You first create a new world in either Minecraft PE or MCEdit, and download a program called iExplorer by macroplant.
If you create the world on Minecraft you'll have to extract the world folder from your iPod by opening iExplorer and navigating to com.mojang.minecraftpe/documents/games/com.mojang/minecraftWorld. Once you've done that you'll need to toss a copy of a player.dat file into the folder so that MCEdit can read the map. From there you just edit it and put it back on you're iPod when you're done.
If you create the world in MCEdit you'll need to make the world specifications 16x16 chunks, once you're done editing copy the entire world and export it as a schematic. Then you'll need to make a world in Minecraft PE and do the same thing with the player.dat, open the world in MCEdit and erase all the blocks, and import the previous schematic.