my skeleton drawing is incomplete but what I've done so far is done correctly to me . With proportioning each piece and making sure it was in the right place I would stop and look at the original picture and measure with shapes and think as if the picture was smaller to make sure my pieces weren't to big to small to close or to far away. I drew and triangle coming from the top of the head to the shoulders to make sure it was angles correctly. With each bone I carefully drew a light sketch and after I was finished I looked and made sure the bones lined up and them hen went over and made them darker and added clone . With adding volume I just shaded in the dark areas and made sure I left what showed white alone . I also had to using different techniques like dots to draw on the top of the skull and lines for areas that just showed dark lines down the bone. I think my proportions and volume are good but I could do better with my proportion even though nothing can be perfect .