Press the controller icon to create a match. Choose from either an Open Challenge, for anyone to accept, or a Direct Challenge, for a specific player.
Choose the game you want to play, the system you'd like to play on, the amount of money you're wagering and the length of the match -- a single game, or best of three. If you have any specific rules you want to play by, enter them in the Custom Rules field.
When you're done, click the Create Match button. You'll be notified when someone accepts your match, and you can start playing!