First, turn on your xbox and open the disc tray. Take out whatever game you have in there and insert modern warfare 3. Then start the game up. Once you are at the main menu, click on the "campaign" tab, and select new game. Sit through the two minute long cutscene, and the achievement is yours.
You can do this on any difficulty, though REAL gamers do it on veteran.
***EDIT: You must first purchase modern warfare 3 before attempting this achievement. If you don't, the achievement will not unlock.***
Will also work if you steal the game. Thanks Ben Online NL!