Despite of the fact that I often do portrait painting from photography, I really must emphasize again, that portrait painting from observation is important. Study as much as you can from a life model sitting in front of you.
Training your drawing skills is fundamental. Accompanying a study I made last Tuesday night. I used Terra Rosa (Winsor & Newton) mixed with a little bit of Burnt Umber (Rembrandt Talens.)
I give you a quote from Bridgman´s “Complete guide to Drawing from Life”
The human head
At first the study of heads should be in the abstract, that is, we should forget everything that distinguishes one head from the other and think of the masses common to aal heads. Heads are about the same size. Each is architecturally conceived, constructed and balanced; each is a monumental structure.