You don't need to capitalize every word. You design the map right there on the program. You can't download it. Though I would suggest using Google Sketchup. It is a 3D designer instead of a 2D designer. Here is Sandbox Here is Foundry
Just to clarify Ace's answer: I know I'm spoon-feeding you but I feel nice today. A stickied thread in this forum Ultimate Sandbox Sketchup Template will help you plan out your maps on your pc before you actually build them, also here is a tutorial thread on Sketchup I just dug up Sketchup Tutorial. That should be all you need to get started. I personally find sketchup to time consuming compared to just drawing out on paper but do whatever you want.
How I donwlod maps from PC to xbx? How I mine for fish? Try Print Screen then open MS Paint and Edit > Paste.