Lay on your back with your feet together on the ball. Keep your toes straight up and your knees lined up with your middle toes. Roll the ball towards your butt and back. Repeat. When you are ready for more of a challenge, lift your butt off the floor into a plank position. Tighten your lower abs and tuck your butt to keep your back from arching. Roll the ball towards your butt and back. Repeat.
Remember to keep your pelvic floor muscles tight, your abs below your belly button, and your glutes tight.