Starting Position: Lie prone (on your stomach) over the top of a properly-inflated stability ball (one that compresses approximately 6" under your body weight) with both feet and hands on the floor, hip- and shoulder-width apart.
Gently exhale, contract your abdominal/core muscles to stiffen your torso and slowly walk yourself forward, lifting your legs off the floor. Continue walking out until the fronts of your thighs or knees are resting on the top of the ball. Maintain a rigid torso aligned parallel with your legs. Retract and depress your scapulae (pull your shoulder back and down) as your reach your end position, with your arms fully extended and hands positioned directly under your shoulders. To help with stability, try to keep your legs straight and squeezed together. It helps to think about squeezing a quarter between your knees.
Curling Phase: Exhale and slowly pull your knees towards your chest, rolling the ball forward as your knees tuck under your torso and your hips move upwards. Continue moving until your knees are positioned under your hips and your tibia (shinbone) lies over the top of the ball. Hold this position briefly.
Lowering Phase: Inhale and push your knees away from your torso, returning your body to the starting position.