What You Will Need:
Xbox 360
Computer
Flash Drive
Modio.exe
USBXTAF

First off, You need to have your account on your Flash Drive.
Plug The Flash Drive into Your PC
When in the PC Open Up USBXTAFXPlORER.

Click File>Open Device
Keep Opening Each Folder You See Until You Cant Go Any More.
Right Click it and Hit Extract.
Extract To a Handy Location.

Then Open Up Modio.
Drag the Recently Extracted File In Modio and it Should Look like This.

Click The Contents Tab in Modio.
You Should See your .GPD Files.
Now it Time For the modding.
Search google For Ex (Cod4 Modded GPDs Or Black Ops Modded GPDs)
Download Which ever One You Choose.
Save it to a Handy Location.
Back to Modio.
Look at the Recently downloaded GPD and Notice the filename.
Look in the Contents of your profile and replace the one with teh same name as the one you downloaded.
Select your modded gpd that you recently downloaded.
once finished, Leave the contents and Click Rehash and Resign.

Open Up USB Xplorer again.
Click all down the folders as before.
When you get to the file, this time delete it and drag in the one you extracted earlier.
Then Your Done!
~~~rmbb4life