When a guy knows his girlfriend would cheat on him, he has one of his friends hook up with her and while they're goin' at it he and about 10 other friends to come in and beat the shit out of her... with bats.
you coerce your ex that cheated on you into a date that will hopefully end with sex, so you have it set up so that as your fucking your girl, you say a secret word and your friends storm the room and beat the living shit out of your lady
First you pick up some random girl at the bar, make sure she is really drunk and bring her back to your room. Then you have all your friends wait outside while you start to go at it with her. As soon as she is about to "go" you yell "now" and all your friends come and beat the shit out of her. Then she wakes up in some ditch the next morning with no idea what happened.
Follow the steps above but make sure you beat her with Wet-Pool Noodles and rubber hoses so it bruises the muscle and not the skin.