AFAIK, you enter a ladder and get challenged or you challenge other player(s) to matches and after the match, you post the scores. I'd keep a camera with you if you plan to play competitively, because a lot of people post incorrect scores because they're little brats and you need proof (a dispute) to get your scores to be corrected. I was #8 on the Halo 3 GameBattles because I had an amazing Win/Loss ratio, but sadly, the summer ended and school became an issue, unlike the 40-year-olds that took the other 9 Top 10 spots haha