Custom biomes are ecological Abominatios Masterpieces that ofen are a standard Equlinox vanila biome (ex. Forest) with a twist (ex. genetic modification or absence/existence of trees, rocks, etc...) .

Less ofen there are Mixes of biomes (ex. glassland + desert = savananhah) wich is this "guide" targeing.

step 2.0
find an Area (preferebly the rigth altitude for the biome you have in mind.).

step 2.1
choose a selection of plants (ex. grass tuffs, desert grass umbrela trees...)

step 2.2
Torture geneticly modify plants to youre likeing (ex. grass tuff become yelow or umbrela tree become large and orange...)

step 2.3 (skip if in creative mode)
wait for the plants to take root and die reproduce.

step 2.4
add animals. and geneticly modifi them.

step 2.5 (skip if in creative mode)
wait for the ecositem to colapse thrive!

Here are some biomes i made my self:
1 ice age.
just combine holy bushes, spruce and fir trees, tall and juniper trees, white colored sage, ligth grey (-50 pecent size ) colored fern and add large (250 percent size ligth brown color) boars (wolly mamoths), big ( 50 percent size dark brown) wolves (dire wolves), white goats ( insert prehistoric mamal name here).

2 savananah
combine tall desert grass, normal grass (just make it yelow) and orange large (200 percent size) umbrela trees and some brown rocks and stones... for the animals add some normal desert hares and (tan colored) goats for antilopes.

3 prehistoric forest glade
add all the normal plants (exept ferns) that exist on the forest floor, then add Tall (200 percent size) wheat (brownie poits when realy edible) and dont forget small golden acer trees. then add larger then normal (grey and slow) chickes (dodoes), and large (dark brown ) guiney pigs (capibaras).

4 baren/radioactive
add barley, turnips, some (dark brown) fly trapers and (dark red-brown ) wobly trees.
the only animal here is the fly (50 percent size and dark-deep red).

5 autum forest (no grass)
add mint, tulips, dayses and carots. add eucaliptus red maples and birch (dont forget to add some warm autum colors) and atummal trees here and there. animals are bees sparows and dark brown or black bunyes.

i hope you liked this "guide" (more of a list to be honest) and if you have any custom biome ideas leave it in the coment secion below. :)