The same way you become a pro at everything - practice.
Draw/paint the stuff from life, and by doing these observational studies, you'll learn how to make them look convincing without actually needing the reference materials.
"Composition, values, shapes, etc.." - without those basics, good luck doing much anything from your imagination. (assuming you're aiming for realism.)
And of course, stressing the absolute importance of PRACTICING.
Doctors heal you, Artists immortalize you.
"Those who can, do. Those who can't, teach" - bullshit.
The usual staples for anatomy: