Get in push-up position, with your hands flat on the ground under your chest at shoulder-width distance. Balance your lower body on the ball, the middle of your shins down to the front of your feet should be on the ball. Don't let your lower back sag, keep your spine in a straight line, and never let your hips dip below your shoulders.
Visualize driving your belly button into your spine, creating a flat back (no butts in the air!) that doesn't cause strain. Hold 20-30 seconds, repeat two times.
Make it more challenging: Try knee tucks, bringing your knees slowly to your chest, 12 to 15 reps. Or you can try plank pikes, lifting your butt straight up in the air, rolling the ball under your feet to your toes. Your legs must stay straight the entire movement, and your head should fall directly between your shoulders.