|Portrait Of Joris de Caullery, by Rembrandt van Rijn|
I originally wrote this post to help an art student with his portrait studies, hence the informal diction. This post concentrates mostly on analysis of form and edge control in the design of the portrait. Without these two, the portrait, and indeed, all compositions, fall apart.
We can only see anything because of light, and form can only be revealed through light.
The light and shadow pattern is a compositional element as much as anything else, and ideally should have a sort of unity to it.
Once you have established the light and dark pattern, utilize the light to model and describe the form, using your basic knowledge of rendering form (ie highlight, light, halftone, shadow core, reflected light, cast shadow)
For example, a rather obscure Dutch artist known as Rembrandt van Rijn painted the portrait shown above:
Notice how although most of the painting, including the figure, is in shadow, the majority of the face is illuminated by light. Notice even more how the light shape is almost a single shape, as is the shadow; this is not a patchwork quilt of values.
Also notice how Rembrandt not only paints more detail in the light, but models the light more; the shadows he left rather thin and flat. This is is a compositional choice that maintains the focus in the light areas and away from the shadows.
And do NOT draw portraits from photo reference unless you have to; always draw from life if possible. (anatomical studies from reference is fine, for obvious reasons ;-))