How do you make Halo 3 helmet?

Fisrt you must make sure you have Xbox Live Gold. Then go to matchmaking, and in any playlist, reach the rank of Private (don't worry, it isn't hard). Then go to settings (press start) and go to appearance, and you should have the ODST Helmet! Congrats! Also, you'll know when you get to private when you get a achievement and get a note.