Tracing the skull photos results in an outline like that faint one you see in the bottom right. The tracing part is to get proportions right more than anything else. I use a small table-top light box; you can get them pretty cheaply from art supply places these days. Before, I was restricted to tracing in the day time, against a window, or I'd hold my papers up to a lamp. That was before I switched to compact fluorescent bulbs and the heat from the incandescents would warp my paper.
Next, I pencil in the detail while looking at the actual skull. Oftentimes the detail hasn't been captured by my photograph. Though I try to light the skull as best as possible, details may be obscured by shadow. Also, the camera lens doesn't necessarily see what my eyes see. For example, the curvature of the back of the skull in the top view blocks the camera's "one-eyed" view of some of the details at the base of the back of the skull that I can see by having 2 eyes set apart from each other. For the top views, I draw one half and then use tracing paper to reflect that onto the other side. That saves a lot of effort.
On the left we have the Brown rat Rattus norvegicus, and on the right, the Black rat Rattus rattus. I'm starting to wish I'd done this Mammal skull illustration series when I was drawing kangaroo rats, which are much more likable, but alas. I do have a mountain lion/ cougar coming up. Anyways, I was going to point out that though to most everyone, a rat is a rat whether it's black or brown, the skulls are actually pretty different.