All the tutorials by drawing with jazza are really good. I learned muscle anatomy from his tutorials.

I'll look them up! Muscle anatomy is something I definitely need to learn.

I asked an artist buddy of mine, and she said to look into colour theory, ambient occlusion and subsurface scattering. I’ve no idea what that means though. And for linework, she says to just know your form, and don’t stress about the lines being perfect or drawn neatly, since you can always fix that. You’ve gotta think of your subject as a 3D object, and that should be clear in the sketch. *shruggo*

Thank you very much for asking about it! I know the concepts, but I haven't really explored them much. I'll do some testing and give it a shot!