Here is a short clip of a few movements you can do using your medicine ball to work out your abdominal muscles.
Try to do each movement 10x and finish out entire circuit of workouts in this video. Once completed rest for a minute or two and do it all again.
Get 3 sets of this work out in. Remember 10x each movement, followed by the next movement another 10 and so on.
I train my abs often. Try to do this 4 days a week.