Index -> Interface -> Casual Lobby -> Launching Matches

When players create or join a match, they enter the Match Launch screen. This screen lists each player's ready or unready status. There is also a chat area to communicate with any player who joins the match. When you are ready to play, click your Ready to go button. Once all players have done so, the match will begin.

To learn more about how to play the match, see the section on Playing the Game.