This may not be the intended answer for the question, but it is something that has worked great for me so I will share it anyway.
How I built my abs was through a workout app developed by Athlean-x called 6-pack Promise. It is a daily exercise program for your full core, no equipment needed. It is worth spending the 5 or 6 dollars for the full version.
The reason why it works so well is it gives short, high-intensity workouts for your ENTIRE core, not just abs. If done with proper form, the 5-7 minute workouts will literally burn your core, a great feeling if I am being honest!
However, if you want exercises specific for the ball, I will offer a couple bits of advice. Put your feet on the ball and lay on the ground, do different variations of situps while bringing your whole body as far up as you can. Don't just do regular, straight situps. Cross over each side of your body to work on the obliques. The reason the ball is so good is that it forces you to balance while exercising.
Hopefully this helps in your journey of a better looking and stronger core!