Reading threw this free book I learned why the original teacher chose casts and statues for these studies. Two words for my fellow furrys. 'Animal Statues'. Find pictures of them in white or marble. They have already been simplified in form. The form will be as easy to follow as a ball. If your new to this, like i am, do small areas first. Don't overwhelm yourself by trying to do the entire thing at once. Do the eye, jaw, mouth or nose. Like the original studies take as much time to get both the shape and shading correct. This isn't an exercise that can be rushed.
As far as taking measurements and getting proportions the video below is the best I've found.
https://www.youtube.com/watch?v=v-PWjlah7dwThis guy has a unique take on painting realistic.