A fitness or stability ball is a great tool for your abs and core muscles. Do it with the following steps; Step 1. Fitness balls usually come in different sizes. You must pick a fitness ball which allows your knees at the right angle. Step 2. Thereafter, sit on the ball and take the support of the floor with your feet. Step 3. Then move the ball with the support of your back and feel the stretch on the abs. Repeat the same for 1 to 3 sets of 12 to 16 reps.