Open Minecraft, and you should see "Flatworld" in your single-player world list.

Looking for a clean slate for your next big project? I've put together a custom superflat world ready for download! Whether you’re testing redstone, practicing your building, or starting a unique survival challenge, this map provides a perfectly level foundation. [Flatworld.zip] How to Install: Download the .zip file from the link above.

Extract the folder inside to your folder in your .minecraft directory (on Windows, press Win + R and type %appdata%\.minecraft\saves ).