You will need:
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2 controllers
2 accounts

Step 1: Be level 55+ on Multiplayer on the account you want to prestige.
Step 2: Download the Save (Credit to UberKitten_1337)
Step 3: Move you Multiplayer progress to a USB and open it in Modio or Horizon.
Step 4: Go to Device Explorer and extract your Multiplayer progress to Desktop.
Step 5: Now, copy the numbers after mpdata_ on your save for your account.
Step 6: Rename the downloaded save to mpdata_(whatever you just copied from your save)
Step 7: Open your save that you moved to your USB and go to File contents.
Step 8: Right Click and replace that file with the newly downloaded SL save.
*Now you are ready to do the actual glitch.*
Step 9: Put the USB back into your xbox and load up Modern Warfare 3.
Step 10: Select the save device with the modified save game on it. (Your USB)
Step 11: Turn on your second controller, but do not sign into an account.
Step 12: Go down to system link but do not click it.
Step 13: With your second controller with nothing signed in click on system link twice.
Step 14: With your first controller click on system link, barracks, prestige mode, and then go through to the last step of prestiging.
Step 15: Sign in with your second controller to an xbox live gold or silver account.
Step 16: Once signed in, press B and then you should see the prestige icon change to your current prestige.
Step 17: It will say you are prestiging to the same prestige, but if you are over level 55, then it will prestige you to the next prestige. Click Prestige.
Step 18: Congratulations, you are now the next level, level 1.

Hope it helped, haven't seen a tutorial for this, as everyone is being stingy with it.
Have fun.

Edit: I could make a video tutorial for this if you guys would like, but i'd need someone to donate a 55+ account, and it would be recorded on an iPhone 4, so not so good quality.. But, it would help people do the glitch.

Vouches: