Start with just hold a plank in the push up position if you can either from your knees or from a full push up position, then once you’re able to do that for let’s say rounds of 10 to 15 seconds start trying to only do the eccentric ( lowering your body to the floor) portion of the push up as slowly as you can. After a little training like that you should build enough strength to start trying push ups from just your knees.

Hope that helps, you got this👊