You just have to find "your" right positioning with your legs or upperbody!First, set up your upper body, lean on the ball, you may want to start with a bent knee ,try that and do a sideline crunch see how that feels you may even have to go higher up on the ball and have more of your upper body into it to add to the resistance! Next level, bring knee into extended positon behind the top extended leg(wrapped by ankle) are you stable??? you may have to scissor your legs open for a wider stance for balance(extended leg on top front an lower leg to the back)You'll find your positioning, you just have to play with it til you are comfortable !Once you mastered the set up you can work on making it harder by taken the one hand off the ball and putting it on your head for an advanced workout! Also the more your upper body is over the ball the harder it is!
HTH;-)