Here is a demonstration of pushups using 3 medicine balls. It’s a great way to challenge the core as it makes the pushup more unstable versus doing it from the floor. I have both feet stacked on one ball and to make it more stable you can use two balls for your feet. 

Keep a neutral spine, abs pulled in, inhale going down and exhale pushing up. Go ahead and try it. Remember, never compromise form so if that’s the case, place hands on the floor and feet on the ball and try it that way.

Good luck!

https://youtu.be/JBGCvkNMAvk