Besides the obvious: studying the fundamentals of basic 3D forms, the other key for the understanding of the human figure is anatomy my dear friend, and more work on observational drawing to aid such studies. Personally I always recommend to complement Stan's Portrait Drawing Fundamentals with some anatomy books. The most complete and well redacted I can think of are "Anatomy For Sculptors" (page 99 on wards) and "Anatomy Of Facial Expression" (the first chapter) by Sandis Kondrats and Uldis Zarins (but any well written book about anatomy for either 3D or 2D artists will suffice). And to aid you in the mental process of coping from reference I highly recommend the Bargue Method as explained by Stephen Bauman on his YouTube Channel. After grasping the basic concepts given by the Bargue Method, and repeating the same copy from diagrams and photos process for each part of the face as an aid to the Proko Portrait Drawin Fundamentals Course you will be able to better understand, recognize and draw the whole head.