Set the bet amount before the round. Air presses are automatic; that is, if one is called, it is automatically accepted.
Say you're on the first tee. Your opponent steps up and smacks his drive, but, uh-oh, it looks like it's headed for the rough. You call an "air press," and the bet is in place. What you're betting is that you'll beat your opponent on this hole. Now you step to the tee. Your opponent has the option, while your ball is in the air, to double the bet.
Air presses can only be called by a player who has not yet hit on a hole (thereby ensuring that the opponent has the option to double the bet).

