This series of tutorials begins with a rendering of a 3D model,
but you could also begin with a photographed human model.

Several of the initial procedures will refer you to specific steps from the "Portrait Postwork" tutorial by Anat Dayag (AKA: "A_") at www.anat.creativedreamers.com. Anat's tutorial presents these particular steps so well, I saw no need to "re-invent the wheel,"
and the basic technique you'll learn will be used
again and again in your postwork.
Links to those steps will open in a new window.