How can you do excercises with a ball?

Depends on what you want to do, a few examples are Ab Curls, the plank and press ups. For ab curls sit on the ball, hands by the head, lean back to approximately 45 degrees and crunch the abs basically tensing them.

For the plank lie on the floor with the ball in front of you, place your forearms on the ball and roll forwards so that your body's straight with your forearms resting on the ball.

Press ups as normal but with feet up on the ball.