I'm self taught so I can only provide advice and my methods:

I'm not sure what program you'd use, but personally, I use Flipaclip on my phone with (clean) fingers. There is a built in frame and layering system. Once the scene is sketched, I separate the body parts of the characters into layers, It'll be easy to move them around as I make the rest of the frames, also keeping it consistent throughout as I'm manipulating the same sketch or so. Stretching, moving, bending, transforming, etc. It also helps to color code or change the opacity of some bodyparts/layers to keep track

Often times I have separate layers for the head, face, body, limbs (depends on how many), private parts, etc. Some of them get merged in the end to make more room. If it's a sketch, while I'm lining the sketch, I will merge the sketch layers as I go and then the linearts will be the separate layers leftover instead.

More about making the animation itself: Use key frames. I tend to have the sketch I started out with, and then I move those parts to make the final frame I want in the final animation, say, the dick fully penetrated or whatever.

Then I like to make two inbetweens, one for the starting frame slightly moving tha dick into it to be placed after the first frame, and one of the final frame slightly moving the dick out to be placed before that final frame. Then make a middle frame to tie those four frames together and boom. 5.

Except the minimum frames I use is like 9 or something, so that's just describing it as simple as possible. Basically fill in more blanks with more inbetweens. Squash and stretch is a good method, or making another frame based on the final one but let's say the dick pushed in more, and putting that before the final frame, to add a bit of weight/force into the thrust.

It also helps while you have the full 3 inbetweens drawn to just move maybe the head or something in the blank inbetween frames you'll draw later using it as a guideline for when you add the rest of the parts to see how smoothly it'll transition.

I'm sorry if none of this makes sense I'm trying it's easier if you see it yourself but you can ask for clarification lol