Index -> Interface -> Casual Games Lobby -> Launching a Game
When players create or join a game, they enter the Game Launch screen. This screen lists each player's ready or unready status. There is also a chat area to communicate with players who have joined the game so far. When you are ready to play, click your Ready button. Once all players have done so, the game will begin.
To learn more about how to play the game, see the section on Playing the Game.