In order to do sit ups on stability ball, what you want to do is lie back, put your hands behind your head. You're not gonna go too far back, you just gonna come to an angle where your neck is inline with your back and then you are gonna come all the way up. See, you go right back down, your hips are gonna slightly move as well when you are doing this. You want to keep your back pressed against the ball as much as you can. And, this is a little bit more basic than a crunch. You are keeping your chin off your chest, your elbows are open, you breathe out on the way up and in on the way down. And, that would be how you do a sit up on the stability ball.