GO IN SPLITSCREEN AND KILL YOURSELF BEFORE DOING ANYTHING
1) Download the Split-Screen modding tool
2) Go to your memory on your Xbox
3) Find Modern Warfare 3 in "Games"
4) Find the Multiplayer Progress save and move it to your USB
5) Plug in your USB into your computer
6) Open up Horizon
7) Extract your MW3 mpdata save to your desktop
8)Open the mpdata save in Horizon
9) Click "Contents"
10) Extract the mpdata_*random numbers* to your desktop
11) Open it in the Split-Screen Modding Program
12) Save once you've finished modding
13) Replace the new modded spdata with the old one in the contents tab
14) Rehash and Resign
15) Finished!