A platform push up on a ball is by using the opposite side of the ball, so this is your platform. You're going to bring you hand, either on top and you want to keep them equal distance apart or you're going to grab on to the handles on the side. What you're going to do is, you're going to bring your chest towards the platform, so you just reach down, keep your neck in line with your back. So you want to keep your eyes forward not looking down.
Then if you want to do a different kind of push up, you can do a tricep push up on this ball. So you would bring your hands closer together again, equal distance apart from the middle, and you're going to bring your elbows in to the sides and then back up and push down, push back up. If you wanted to modify this you can do either the tricep ones or the regular push ups on your knees. So you would just push down and back up. You want to try to keep the BOSU level. You don't want to dip it too much towards you, so you want to try to keep it back in the middle.