Lie on your back, holding the ball in your hands overhead. Raise your legs straight up to 90 degrees. Now raise the ball up to pass it onto your legs. Grip the ball between your feet and lower your legs till they are about 30 degrees off from the floor. Lift the legs up again and pass the ball to your hands. Take your hands back to the overhead position and repeat.

Every time you perform a lifting move, you will be using yoru abdominal muscles. Contract them hard.