Introduction

Biomes have been present in Minecraft since the Halloween Update, and the Biome ID is included in the Anvil file format since Minecraft 1.2. The Biome ID for worlds stored in the older McRegion file format is calculated on the fly, which could lead to a discrepancy between the earlier generated world, and the features (such as grass color, snow) based on the calculated biome IDs.

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Biome IDs

Ocean - 0
Plains - 1
Desert - 2
Extreme Hills - 3
Forest - 4
Taiga - 5
Swampland - 6
River - 7
Hell (Nether) - 8
Sky (End) - 9
Frozen Ocean - 10
Frozen River - 11
Ice Plains - 12
Ice Mountains - 13
Mushroom Island - 14
Mushroom Island Shore - 15
Beach - 16
Desert Hills - 17
Forest Hills - 18
Taiga Hills - 19
Extreme Hills Edge - 20
Jungle - 21
Jungle Hills - 22

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Creating a Custom Superflat World

Okay, create a new world.
In the world options, switch the world type to Superflat and click Customize. There, click Presets. Now delete the text in the box at the top and I'll show you what you can do with the basics.

Tips:
Do NOT go over layer 255. Or Minecraft will crash.
The order of layers is from layer 0 to layer 255(left to right).
The last statement does not need a semicolon at the end.
Block ID List: Here

Now let's get started!
Type in the box to start 2;. This is the version number, so if this gets updated, you can still use this code.
Next, after the ;, type an id (Ex. 7(Bedrock)). Don't type a semicolon this time; instead, add a comma for an extra layer.
Now here is how to make a block in multiple layers. Let's say you want to make 2 layers of dirt. Type axb,. a is the amount of layers the block will have, and b is the block id(0 - 255). We're done with that.
We're going to top it off with a grass block layer(yes, we're making Classic Superflat). This time type 2 and close it with a semicolon. It should look like "2;7,2x3,2;".
After that code, type a biome ID then end it off with a semicolon.


Then add your decorators.
Here is a list of some of the fun ones:
biome_1: Ores spawn! (Note: Stone must be in the preset.)
village: Villages will spawn in your world.
stronghold: Strongholds will spawn in your world.
mineshaft: Mineshafts will spawn in your world.
lake: Water lakes will spawn in your world. Try adding the prefix lava_ for lava lakes.
decoration: Biome decoration will spawn(Ex. Trees, tall grass, etc.)
dungeon: Dungeons will spawn in your world.

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Final Result

So, now all of the different parts of the code are complete, we need to piece the whole code together. Here's an example of a completed code (still color coded):

2;2x49,46,2;1;dungeon,stronghold(count=3),decoration

Which means: version 2; 2 layers of obsidian, 1 layer of TNT, 1 layer of grass; biome 1 (plains);spawn dungeons, 3 strongholds and decorations.