Besides normally generated maps, Minecraft offers the option to create so-called superflat maps. By using a preset code this map type is highly customizable. Unfortunately, there is no documentation on how to write such a code, and in-game customization is very restricted as of now. All you can do is choose from a set of a preselected codes and remove some of their layers.
Using Superflat Generator, however, you can customize every single aspect of it. And once you're done, you can simply copy the code into Minecraft to create some awesome and unique superflat worlds.
When you first start the app, the default "Classic Flat" preset will be loaded. You can see the current superflat code of the preset in the code field at the top. You can either change this setup manually by editing the code field, or you can use the controls below to change it.
By default, the "Layers" tab will be active. Here, you can add a new layer by selecting a block, searching for it or by entering its block id. Once added, you can change the height of the layer by using the number input on the left. If the block can have different meta values, you can click on the layer to expand the options. Finally, the layers can be removed, and you can reorder them with drag&drop.
By clicking on the "Biome" or "Features" buttons, you can further customize the superflat setup. Does not need any explanation I think. You can hover over most labels to see some additional info.
Once you've finalized your superflat setup, you can simply copy the resulting code (Hint: Ctrl + A then Ctrl + C ) and paste it into Minecraft ( Ctrl + V ). Here's how to find the input:
Single Player -> Create New World -> More World Options... -> World Type: Superflat -> Customize -> Presets
Additionally, with the app, you can
- click on "New Preset" and give it a name to reuse it on this page
- bookmark the current URL to reload it later
- share the current URL