In order to mod your must have: computer,USB(1GB at least), Horizon, MW3, and Modded gamesave.
1. Download Modded gamesave [ Register or Signin to view external links. ]
2. Make a profile on your USB and start up MW3 campaign
3. Wait for all the cut scenes to be done and when it says "Checkpoint Reached"
4. Now save and quit and unplug your USB and go to the computer
5. Open up horizon and drag your MW3 save game into the middle
6. Now drag the file I provided right next to your game save on horizon
7. Copy and paste all your ID's to the modded game save
8. Now Click save, rehash, and resign on the modded game save
9. Then over right your save game on your profile
10. Go back to the xbox and resume your campaign
11. When you spawn you should have mods
12. You can play either Campaign or Spec Ops with the mods

Mods include-
Campaign:
-Aimbot-
-Wallhack-
-Super Jump-
-Colored Message-
-Compass size-
-Cartoon Vision-
-9x Speed-
-GodMode-
-Slight of hand-
-Steady Aim-

Spec Ops:
-Round Skip-
-GodMode-
-Steady Aim-
-Slight of hand-
-9x Speed-
-Cartoon Vision-
-Pro Mod-
-Colored Message-
-Compass Size-

I hope this tutorial will help out a lot of you. I will be making a lot more tutorials on games, so if you want a game reply the game you want mods on!