How does one create a demiplane in D&D? There are scores of references to such, both in the literature and in modules, stretching all the way back to 1st edition. I assume that the original answer was "via DM fiat", accompanied by appropriate hand-waving. But, since "DM fiat" is no longer politically correct, as of V3, and there are now various rules to address it. In particular, I am aware of the following:

• The demiplane seed power from The Manual of the Planes (a V3.0 sourcebook), available at Planeshifter 10.
• The genesis psionic power from the Expanded Psionics Handbook. (SRD)
• The genesis spell, from the Epic Level Handbook. (SRD)
• The genesis spell from Deities and Demigods. (This does not appear in the "Additional Spells" section of the Divine rules in the SRD, presumably because it is essentially identical to the one from the ELH, which does appear there.)
• The create demiplane, lesser, create demiplane, and create demiplane, greater spells from Pathfinder. (PRD)

I'll copy all of above here, to facilitate discussion.

From Manual of the Planes:

Demiplane Seed: The demiplane seed is a small pocket dimension grown by a planeshifter who has reached 10th level. This pocket dimension is often used as a base of operation. A planeshifter can own only one demiplane at a time, and cannot construct a new one unless all portals to the old one are destroyed. The planeshifter must have a single flawless gemstone of at least 1,000 gp value and work on the demiplane for 100 consecutive days, for 8 hours per day.

Upon completion of the work, the seed opens into a minuscule spherical demiplane, 1 foot in radius. It grows quickly, gaining 1 foot in radius per day up to a maximum radius of 10 feet x the planeshifter's highest caster level. After reaching that size, the demiplane continues to grow slowly, gaining 2 feet of radius per year. If its creator perishes, the demiplane stops growing.

The planeshifter's demiplane has the following planar traits: normal time, alterable, and normal magic. At creation, the planeshifter can set whatever gravity trait, elemental traits, and alignment traits he likes, and can choose whether the plane is finite or self-contained. While the demiplane isn't morphic, the planeshifter has total control over the landscape at the moment of creation, so he can decide whether it will be a foreboding, mountainous wasteland or a bucolic forest. The terrain set at creation extends itself as the demiplane grows. While it's not possible to fit jagged peaks into a 1-foot sphere, cliffs and summits appears as the demiplane grows.

The demiplane grown from the seed is unfurnished, so the planeshifter must provide what construction is necessary. It has a single portal entry, which the planeshifter may control for access. Demiplanes are often used as hiding places, research labs, and prisons for particular beasts.

From the Expanded Psionics Handbook (file PsionicPowersG-P.rtf in the SRD):

Genesis
Metacreativity (Creation)

Level: Shaper 9
Display: Material
Manifesting Time: One week (8 hours/day)
Range: 180 ft.; see text
Effect: A demiplane coterminous with the Astral Plane, centered on your location
Duration: Instantaneous
Saving Throw: None
Power Resistance: No
Power Points: 17, XP

You create a finite plane with limited access: a demiplane. Demiplanes created by this power are very small, very minor planes. This power works best when manifested while you are on the Astral Plane. Manifestation of this power creates a local density fluctuation that precipitates the creation of a demiplane. At first, the fledgling plane grows in radius at a rate of 1 foot per day to an initial maximum radius of 180 feet as it rapidly draws substance from the surrounding astral ectoplasm. Once the new demiplane reaches its maximum size, it doesn't really stop growing, but its growth rate decreases to only 1 foot per week (approximately a 50-foot increase in radius per year). Once your demiplane is created, you can travel to it using astral caravan, plane shift, or some other power or permanent link that you arrange for separately.

You determine the environment within the demiplane when you manifest genesis, reflecting most any desire you can visualize. You determine factors such as atmosphere, water, temperature, and the general shape of the terrain. This power cannot create life (including vegetation), nor can it create construction (such as buildings, roads, wells, dungeons, and so forth). You must add these details in some other fashion if you desire. You can't create lingering psionic effects with this power; you have to add those separately, if desired. Similarly, you can't create a demiplane out of esoteric material, such as silver or uranium; you're limited to stone and dirt. You can't manipulate the time trait on your demiplane; its time trait is as the Material Plane. Once your demiplane reaches 180 feet in radius, you can manifest this power again to gradually add another 180 feet of radius to it, and so on.

Antigenesis
If genesis is manifested on the Material Plane, the power takes effect and the demiplane begins to grow at the rate noted above, but it gets no larger than a radius of 1 foot per level. The energies of the new plane are exactly canceled by the energies of the original plane, creating a dead spot like a limited cancer on the original plane. The expanding boundary of the dead spot wipes away all construction, crumbles natural land forms, and evaporates water, leaving behind a uniformly level area of inert dust. Living creatures that pass the boundary of the growing dead spot are not directly affected, but plants can find no sustenance in the dust of the dead spot, water-breathing creatures die quickly when water turns to dust, and mobile animals know enough to leave the area alone. Once the wave of change passes, no special essence remains in the dead spot, and it may be colonized naturally over the course of several years by bacteria, plants, and animals.

XP Cost
1,000 XP.

From the Epic Level Handbook (file EpicSpells.rtf: in the SRD):

NON-EPIC SPELL

Genesis
Conjuration (Creation)

Level: Sor/Wiz 9, Creation 9
Components: V, S, M, XP
Casting Time: 1 week (8 hours/day)
Range: 180 ft. (see text)
Effect: A demiplane coterminous with the Ethereal Plane, centered on your location
Duration: Instantaneous
Saving Throw: None
Spell Resistance: No

The spellcaster creates a finite plane with limited access: a demiplane. Demiplanes created by this power are very small, very minor planes.

A character can only cast this spell while on the Ethereal Plane. When he or she casts the spell, a local density fluctuation precipitates the creation of a demiplane. At first, the fledgling plane grows at a rate of 1 foot in radius per day to an initial maximum radius of 180 feet as it rapidly draws substance from surrounding ethereal vapors and protomatter.

The spellcaster determines the environment within the demiplane when he or she first casts genesis, reflecting most any desire the spellcaster can visualize. The spellcaster determines factors such as atmosphere, water, temperature, and the general shape of the terrain. This spell cannot create life (including vegetation), nor can it create construction (such as buildings, roads, wells, dungeons, and so forth). The spellcaster must add these things in some other fashion if he or she desires. Once the basic demiplane reaches its maximum size, the spellcaster can continue to cast this spell to enlarge the demiplane, adding another 180 feet of radius to the demiplane each time.

Material Component: A crystalline sphere
XP Cost: 5,000 XP.

From the Ultimate Magic section of the Pathfinder Reference Document:

CREATE DEMIPLANE, LESSER
School conjuration (creation); Level cleric 7, sorcerer/wizard 7, summoner 5, witch 7
Casting Time 2 hours
Components V, S, F (a forked metal rod worth at least 500 gp)
Range 0 ft.
Effect extradimensional demiplane, up to three 10-ft. cubes/level (S)
Duration 1 day/level
Saving Throw none; Spell Resistance no

You create a small, finite demiplane. You must be on the Astral or Ethereal Plane or on a plane that has access to one of those planes (such as the Material Plane) to cast this spell.

When you cast the spell, you decide whether the demiplane is within the Astral or the Ethereal Plane. It is filled with air or water (decided by you). The plane is generally flat and featureless, such as an earth, stone, water, or wood floor. The "walls" and "ceiling" of the plane may appear like solid earth, stone, wood, or water, or they may end in mist, a featureless void, or a similar unreal-looking border. The plane's environmental conditions are those of a temperate spring day on the Material Plane. You determine the plane's light level (bright, normal, dim, or darkness), which affects the entire plane.

There are no native creatures or plants on this plane, though you may bring some there (if the plane's light is bright or normal, it counts as sunlight for growing plants). The environment of the plane counts as normal terrain for the purpose of effects that target earth, stone, wood, and so on. For example, you could use move earth to create a hill or wall of stone to create a barricade.

When you finish casting this spell, you may bring yourself and up to seven other creatures to the plane automatically by joining hands in a circle. The demiplane is another plane of existence, and therefore is outside the range of any spell or ability that cannot affect or reach other planes. Creatures can only enter the plane by the use of planar travel magic such as astral projection, etherealness, or plane shift. You are considered "very familiar" with your entire demiplane.

As a standard action, you may eject a creature from your demiplane. The creature may resist with a Will saving throw. An ejected creature goes to the closest plane to your demiplane (usually the Astral Plane or the Ethereal Plane, but if you cast this spell on the Material Plane, the creature is sent to the Material Plane). When the spell ends, the plane dissolves, and all creatures in the plane are ejected in this manner with no saving throw. The plane cannot be dispelled, but a creature on the plane can destroy it by using limited wish, mage's disjunction, miracle, or wish and making a successful dispel check.

If you are within the demiplane, you can add to its area by casting the spell again. Alternatively, you may cast this spell again to reset the duration of an existing area to that of your latest casting. If the duration on one area of the demiplane ends and other parts remain, creatures in the expiring area are shunted to remaining areas. If a collapsing portion of the demiplane would leave one section cut off from other sections of the demiplane (for example, if there were three areas connected in a straight line and the center part expired), the stranded sections count as separate demiplanes under your control. You may reconnect these stranded sections by casting the spell again to create a linked area between the two.

You can make this spell permanent with the permanency spell, at a cost of 17,500 gp. If you have cast create lesser demiplane multiple times to enlarge the demiplane, each casting's area requires its own permanency spell.

CREATE DEMIPLANE
School conjuration (creation); Level cleric 8, sorcerer/wizard 8, summoner 6, witch 8
Casting Time 4 hours
Duration 1 day/level or instantaneous (see text)
Effect extradimensional demiplane, up to 10 10-ft. cubes/level (S)

This spell functions as create lesser demiplane, except the area is larger and you can add more features to the plane. You can use this spell to expand a demiplane you created with create lesser demiplane (you do not need to create an entirely new plane using this spell), in which case it has a duration of 1 day/level. Alternatively, when cast within your demiplane, you may add (or remove) one of the following features to your demiplane with each casting of the spell, in which case it has an instantaneous duration.

Alignment: Your plane gains the (mildly) chaos-, good-, evil-, law-, or neutral-aligned alignment trait (see Alignment Traits). You cannot give your demiplane an alignment trait for an alignment you do not have.

Bountiful: Your demiplane gains a thriving natural ecology, with streams, ponds, waterfalls, and plants. The demiplane provides enough plant-based food (nuts, grains, fruit, fungi, and so on) to support one Medium creature for every 10-foot cube of the demiplane. The demiplane does not have any animals unless you transport them there, but the ecology can sustain itself for as long as the demiplane exists without requiring watering, gardening, pollination, and so on, and dead organic material decays and returns to the soil in the normal manner. If your demiplane has ambient light, these plants are normal, familiar surface plants; if it is a realm of twilight or darkness, these plants are fungi and other plants adapted to near-darkness or underground locations.

Elemental: Your plane gains the air-, earth-, fire-, or water-elemental dominant trait (see Elemental and Energy Traits).

Gravity: By default a demiplane's gravity is normal and oriented in one direction, like what most creatures are used to on the Material Plane. By selecting this feature, the plane's gravity is heavy, light, none, objectively directional, or subjectively directional (see Gravity).

Seasonal: The demiplane has a seasonal cycle and a light cycle, usually similar to those of a land on the Material Plane, but customizable as you see fit (for example, your demiplane could always be winter, day and night could alternate every 4 hours, and so on).

Shape: By default, the demiplane has a fixed shape and borders. By selecting this feature, you may make your plane self-contained so it loops upon itself when a creature reaches one edge (see Shape and Size). You may designate areas or locations on the edges of your plane where this occurs (such as a pair of secret doors or a path in the woods) or apply it to the entire plane.

Structure: Your demiplane has a specific, linked physical structure, such as a giant tree, floating castle, labyrinth, mountain, and so on. (This option exists so you can pick a theme for your plane without having to worry about the small details of determining what spells you need for every hill, hole, wall, floor, and corner).

You can make this spell permanent with the permanency spell, at a cost of 20,000 gp. If you have cast create demiplane multiple times to enlarge the demiplane, each casting's area requires its own permanency spell.

CREATE DEMIPLANE, GREATER
School conjuration (creation); Level cleric 9, sorcerer/wizard 9, witch 9
Casting Time 6 hours
Components V, S, F (a forked metal rod worth at least 500 gp)
Effect extradimensional demiplane, up to 20 10-ft. cubes/level (S)

This spell functions as create demiplane, except the area is larger and you can add more features to the plane. You can use this spell to expand a demiplane you created with lesser create demiplane or create demiplane (you do not need to create an entirely new plane using this spell), in which case it has a duration of 1 day/level. Alternatively, when cast within your demiplane, you may add to your demiplane (or remove from it) one of the following features (or any of the features described in create demiplane) with each casting of the spell, in which case it has an instantaneous duration.

Energy: Your plane gains the (minor) negative- or positive-dominant energy trait (see Elemental and Energy Traits). A plane cannot have both the negative-dominant and positive-dominant energy traits.

Magic: Your plane gains the dead magic, enhanced magic, impeded magic, or wild magic planar trait (see Magic Traits, GameMastery Guide 187). If you selected dead magic, you are trapped within your plane unless it has a permanent planar portal (such as the portal feature, below). If you selected enhanced or impeded magic, choose one type of magic to be enhanced or impeded, such as "effects with the fire descriptor or that manipulate fire" or "death spells and spells from the Death or Repose domains." A plane cannot be enhanced and impeded for the same kinds of spells.

Morphic: You may use move earth at will in your demiplane at one-tenth of the spell's normal casting time, and can reshape normal plants in the same manner (such as by twisting trees into a fence or humanlike shapes). You are even able to affect rock formations with this ability, though the casting time for this is only half normal.

Portal: Your demiplane gains a permanent gate to one location on another plane, which can only be used for planar travel. This location must be very familiar to you. This gate is always open and usable from both sides, but you can secure it using normal means (such as by building a door around it).

Time: By default, time passes at the normal rate in your demiplane. By selecting this feature, you may make your plane have the erratic time, flowing time (half or double normal time), or timeless trait (see Time).

You can make this spell permanent with the permanency spell, at a cost of 22,500 gp. If you have cast create greater demiplane multiple times to enlarge the demiplane, each casting's area requires its own permanency spell.