Instead of doing them on your knees, find a sturdy box, chair or other elevated surface to perform this modification — the taller it is, the easier the movement will be (and vice versa). Place your hands on the box so they are higher than your feet. Start doing push ups that way. It's just like a regular push up -- tight core, tight quads. You are just at an incline (hands higher than feet)

A couple of other things may help. Do negative push ups. Don't worry about doing the "up" part. Just slowly lower yourself to the floor with a count of three. Then bring your knees up and get back into the top of the push up position and do it again.

And finally, plank push up position as opposed to a plank on your elbows. Get into the start of a push up and hold that plank position. That's good for your shoulder as well. Focus on keeping things tight -- your core, your quads and try to push your chest a little away from the ground (no sagging).

Good luck.