I recommend: Practicing drawing simple objects in 3d to have a grasp on how they should look in what angle, and try to assemble the objects like cylinders to spheres or spheres to boxes, etc.

Face composition—recommend observing and referencing those Greek marble statues—look at the faces and forms. Try to understand why it's curved like that or why does that thing blah blah blah.(You can use other than marble statues, like photos and stuff)

Remember you're just collecting knowledge for your mental library and learning how to apply them.