Here's the process for my latest figure painting which you can check out here: [link]
The first step is just establishing the gesture and proportion of the figure, the main focus is on the pose.
The second step is to find all the bones so the landmarks for attaching muscles are clearly visible.
The third step is to add on all the muscles on top considering each one individually and how they function as a whole.
Fourth step is just dropping the opacity.
Fifth step is a clean line drawing on top with flesh and fat added in to the mix, so most of the musculature disappears but its used as a guide.
Sixth step is to establish the basic shadow patterns to clearly separate light and dark.
Seventh step is to add a gradating midtone to establish which part of the lights will be the very lightest.
And finally the eighth step is all the subtle rendering to make it look realistic. It looks like a big jump from step 7, but really all it is is adjusting the edges between the lights and the darks and adding in the subtle halftones. It's not hard, it just takes a lot of time and careful observation.