You can find quite a bit of information here:http://www.lucasforums.com/forumdisplay.php?forumid=324
Although this applies to the PC version, much of it applies equally to the xbox. The best way to mod for the xbox, is to create an override folder like so:
You can drop modified files, scripts, textures, etc in that folder and they'll work in the game. It would take a whole tutorial to go over everything - which I'm working on - but this should lead you in the right direction.
Along with the tutorial, I'll be providing several items that will help considerably while modding/testing the game. These items are modified armbands that allow you to do the following:
Add/Change 2nd class
Teleport to any area of the game
Add/Remove NPC's (including custom characters)
Equip hide/claw items
Load custom items and inventory scripts
... to name a few.