Instructions
Place your Halo 3 game disc in your Xbox 360 console while connected to Xbox Live using a Gold Membership account.
Scroll down to the Matchmaking menu after the game has completed loading and press "A." This will take you into the Matchmaking Lobby.
Scroll up on the left-hand menu of the Matchmaking Lobby until you reach "Playlist" and press "A." This will take you to the Select Matchmaking Playlist menu.
Choose any of the Ranked Playlists. It must be a Ranked Playlist, and you cannot change playlists before getting the achievement or your experience will be dispersed among multiple playlists. You will automatically return to the Matchmaking Lobby after you have selected your Ranked Playlist.
Initiate the first match by scrolling down to "Start Matchmaking" and pressing "A."
Play multiple Halo 3 multiplayer matches without changing the playlist until you have satisfied the following criteria: 1) You have achieved a Lieutenant rank; 2) You have 50 experience points; 3) You have reached a minimum level of 10. Ranks, experience points and levels are upgraded gradually after successfully completing a match. You cannot quit the match early. When you have satisfied these three criteria, the achievement will automatically unlock, which will be indicated by the Xbox 360 achievement icon that will appear at the bottom center of the screen.