Answer:
These steps will show you the best way to CREATE an awesome map from scratch!
(YOU WILL NEED XBOX LIVE TO DO THIS!)
1: Go to the XBox Live Marketplace
2: Download the map "Foundry"
3: Start the game up
4: Go to "Forge" mode
5: Choose the map "Foundry"
6: Take the time to delete EVERYTHING in the map that you can possibly delete (THERE SHOULD BE ALOT! AND A BLANK SLATE SHOULD BE LEFT AFTER YOU ARE DONE WITH THIS! OR A LEAST A HUGE AREA! You Can Go On Bungie.net And Find Blank Maps For All The Maps Out There!)
7: Add every thing you want to make a complete map from scratch
8: When you are done with the map go to the pause menu
9: Choose "Save as new map"
10: Name it whatever you want to name it!
11: Try your new map out in whatever you want to try it out in. (If does not show up on the map list you might have to choose foundry and then it should show. Otherwise you possibly screwed up.)
12: THAT'S IT! ENJOY YOUR NEW MAP! CREATED BY YOU!