Lie on the ball, positioning it under the lower back.
Cross your arms over the chest or place them behind your head.
Contract your abs to lift your torso off the ball, pulling the bottom of your ribcage down toward your hips.
As you curl up, keep the ball stable (i.e., the ball shouldn't roll).
Lower back down, getting a stretch in the abs, and repeat for 1-3 sets of 12-16 reps.