Just thought I'd show some of the process for this cover. I begin with the traditionally done pencil sketch submitted to Marvel for approval, then I work on the values and lighting in grayscale, and finally I add color and texture. In this particular case, with just a single character close-up, it wasn't very painstaking.
I adjust and tweak the structure as I paint. If you have the luxury of time, you can assess flaws much more objectively by setting the work aside and not looking at it for a few days.