Slaps are generally more of a fingertip affair (or 1st joint max.). Your knuckles should barely come in touch with the head surface, if at all; at least you don't emphasize this point of contact.
Releasing the pressure and not "grabbing" the slap is important.
I usually start practicing and teaching open slaps. Once you can get that one, correct closed slaps become a toy game. Otherwise you are tempted to deceive yourself and fall into a habit of improper technique. That's my experience.
I have not met one student who did not have pains at first, in the try-and-search period. When I started playing, I played closed slaps (1979) and didn't have less pain. I guess you can't avoid it. But you can overcome this period by using proper technique (acceleration from wrist turn) and finding the right spots on your hands
and on the drum. The level of the wrist position in relation to the head surface is also important. Then there is a number of common mistakes that may occur ... not really mistakes, but disadvantages that will give you a hard time, where others just enjoy drumming along.
I'm afraid it's too difficult to describe what you should do and what you shouldn't without you being present. It's hard enough to explain that to a student in a live lesson! It's a physical experience at last, a body memory thing. Some people develop a nice slap within a couple of weeks, others struggle for several months. There are so many motoric parameters working together at the same time that it wouldn't make sense to even try to instruct each involved muscle fiber. So the last part of the way you would have to go by yourself, anyway.
I think, on the long run you got to go find a teacher, if only for a limited number of lessons first. If you can't afford that, you must afford a long and painful time instead - which is also valuable, so you get to know your drum and yourself better, albeit the hard way
When I started, there was no YouTube; not even home videos ... I'm sure you will find your way, too - sooner or later.
Thomas