Basically do press up related work!
Depending on where you are starting-
work up to 3 sets of 10-12
wall press ups
box press ups
when you can do those
2-3 sets of 8
modified eccentric press up (lowering phase of knee press up all the way to the floor as slowly as you can)
when you get to 3 sets of 8 move to
2-3 sets of 6-8
full eccentric press ups- same as above but on your toes
when you can do those, start adding a 2-3 second pause right at the bottom when your chest is 1 inch from the floor- push back as hard as you can! From here, you should start to be able to push up after a bit of practice.
Form is really important, make sure your hands are strong, that your chest is landing between them and that they aren’t too far forward under your shoulders. Elbows close to sides and not flared out.
It takes time, and practice 2-3x a week, but I’ve had great success with clients with the above program- good luck!