How do I perform an arms overhead ball crunch?

Lie on a stability ball with the ball placed under your low back, bend your hips slightly and keep your knees lined up directly over your ankles. Your feet should be placed flat on the floor, hip width apart. Extend your back over the curve of the ball and extend your arms overhead. Draw-in your navel and contract your glutes. Crunch your upper body forward, keeping your upper arms by your ears and be careful not to move your hips off the ball. Return to the starting position and repeat.