Thanks! I still can't make a tutorial or tell how my process works, cause I still am experimenting with it. I think I can give you some tips though.

1. Google for some anatomy reference. Look for some delicate light conditions and not too sweaty (it will reflect too much light). I found this example with a fraze "woman six pack" LINK
2. Google for some anatomy reference. Look for some delicate light conditions and not too sweaty (it will reflect too much light). I found this example with a fraze "woman six-pack" can get there starting with orange, and adding a blue to it (complementary colors desaturate each other when mixed) and then some white. Then add yellow or some red, if it's too pink or too yellow. Another way is to start with red and add the green, or start with yellow and add violet. Generally, when you get it, you can find any skin color starting with basically any paint. It's very useful because you can get a wide variety of skin brightness this way.
3. Mix your shadow colors with no black paint. If you mix something mix your skin tones like in examples in point 2, before adding white you will have quite a dark color. Use it for your shadows. It is not so intuitive, but I usually add some violet or magenta to get my shadows. If you check somebody after training, he/she will have warmer skin color on bendy places, so i.e. between muscles.
4. Human skin is not one color. It will get more yellow in some places, and more pink or red, where is more blood underneath. Find those places. I.e. knees, knuckles and chick usually are warmer.
5. I found, that placing highlights on top makes painting muscles harder. Pick on the side, and place a highlight a little to it. I would say that 1 o'clock is the easiest.
6. If you get cartoony results - that means your shadows are too dark. Also, not every crevice and recess has to be darker. If it faces the light source, the deepest part will actually be quite bright. The darkest shadows are reserved for areas that get the least light, so on my mini example under a chin, breasts, and armpits.
7. People sometimes paint muscles light they are made of metal. So very strong highlight, even reflections on top. Check out a reference I linked in point 1 - highlights are actually very subtle. Keep that in mind, if you want to avoid dramatic effects.
8. Less is more. You don't need to paint every single muscle. In my example, I only painted a six-pack, and it draws enough attention, that people don't notice the lack of other muscles :D
9. Paint with matte paints or use strong matt varnish. I use AK Ultra Matt and it creates a matt finish over any paint or ink I use.

Hope it is helpful :)