THIS ACHIEVEMENT IS UNOBTAINABLE AS OF FEBRUARY 2010
Go to the main menu and go to "Online". From there you'll see "Lobbies". Once in Lobbies, select the region you want to enter. Once inside you'll have a list of Gamertags in the room. Select one and hit A. From the drop down menu, invite them to your game. Select what kind of game you would like to play, and then go down to Invite. At this point a game invite will be sent. Play the game to the end, and after the buzzer the achievement will unlock. This achievement can be combined with the other ranked game achievement.