Begin by positioning yourself as if you were going to do a push up, with an exercise ball raising your legs off the ground. Your feet should be hanging just over the edge of the ball. Keep your body straight as you bring your knees towards your chest.Roll the ball a few inches forward with your ankles as you bring your knees into your chest, then return to the starting position. This is a challenging exercise because it is hard to keep your center of gravity equal with your legs on the exercise ball, especially when you start to roll the ball forward. Try a few sets with short reps until you are comfortable with the movement.