Crunches have always been a great mobility exercise (and probably the most popular) to strengthen the abdominals.
Using a fitness stability ball challenges your stabilizing muscles, therefore increasing your base strength level.
Start in the most stable position, lying on your back with your legs resting on the ball to perform crunches.
Increase difficulty by having your heels barely touch the ball with your knees and hips bent at a ninety degree angle. As you crunch, don’t let the ball move or wobble at all!
Keep your neck in a neutral position and lift from your chest. You can make any of these exercises easier by bringing your arms down by your side, or across your chest.
Continue to challenge yourself by using more of the stability ball.
Try a supine position on the ball with your feet wider than your hips, and hips positioned lower on the ball (example in the top left corner).
Position your hips higher over ball to continue to increase the difficulty (in top right corner).
If you are successful with those two exercises, then bring your feet closer together (more unstable), then try one leg!