First off, don't use these mods for your first playthrough; it's an excellent game and it deserves to be played through properly. :)
1. Grab the user.cfg file from your Xbox HDD or Memory Unit
2. Open up user.cfg in a con manager (I used Le Fluffe) and extract user.cfg to a folder of your choice (call it extract for the sake of this tutorial).
3. Now open the extracted user.cfg in a text editor and add any variables you wish to :) I'm still looking through the xex for ones to play with, but here are some that I can confirm that work.
Variables can be set to a positive or negative state by either using "1" & "0" or "on" & "off".g_god 1 (invincibility)
g_unlimitedammo 1 (self explanatory)
g_global_god on (makes enemies invincible, though I wouldn't know why you'd want to use this)
4. Once finished adding the variables, save the file.
5. Open up your con manager, open the main user.cfg and replace the older inner (3kb) user.cfg with the one you just modified.
6. Now use the fix/rehash/verify function in your con manager to recalculate the blocks.
7. Now replace the user.cfg file on your xbox HDD/MU with your newly modified one (at least 60kb in size).
If you don't follow this tutorial correctly, the game will automatically generate a new user.cfg file which won't contain any of your modifications.
Here's a vid of the mods in action.
Credit Goes to novax