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.
Upward Phase: Gently exhale, contract your abdominal/core muscles to stiffen your torso and slowly walk yourself forward until your hips lie directly over the top of the ball, lifting both legs in unison off the floor until they are parallel with your torso. In this position, your body weight is distributed over the ball and supported by both hands placed on the floor. Press and hold your legs together so they move at the same time. Think about squeezing a quarter between your knees to help maintain the position.
Lowering Phase: Inhale and slowly lower both legs in unison back towards the floor while keeping them fully extended.