If you have the opportunity to paint from observation you should do so as much as possible.
In this short video I only use raw umber. ( Oil- Rembrandt / Talens )
To begin with, I give the entire canvas a very thin wash of raw umber with standard painting medium.
Then, using a flat synthetic brush I put down the first contours. I determine where the eyes, nose and mouth are. I use a light source above to cast strong shadows under the nose and mouth which are clear markers. They enable me to get a grip on the whole shape. I say this to make it clear that shadows are indispensable. Front lighting is more difficult. It causes unnecessary problems because all the useful shadows disappear. (The same thing happens with flash photography.)
After I have roughly marked down the main structure of the face I can elaborate by adding details. To create areas of light I wipe off wet paint with a cotton swab.
The whole session lasted one hour with two very short breaks.