This is a demo I did for painting a portrait. The model is my favorite, my wife. First I draw an oval shape to fit the face in. I draw a line across the center. The eyes will fit on it. Another one is drawn from top to bottom in the center. The nose will be put around it. Halfway between the eye line and the chin will be another line. This is where the bottom of the nose will go. I draw another line, this time halfway between the nose line and the chin. This is the mouth line.
I now know where everything goes. Next, I start drawing shapes of shadows or values — the area over the brow, the shadow under the nose, the upper lip, under the chin. I mix three shades of flesh color, one dark, one medium, one light. I find the dark areas and paint them in: brow, under chin, upper lip, under nose, etc. Next I find the lightest areas and do them: the upper part of the cheek, the part of the forehead and chin facing the light. The rest is painted in with the medium.
I now use other colors as needed. Lips, eyes, hair, etc. The lip is the shiniest part of the face, so I leave a highlight on it, as well as the tip of the nose.
These are the basics.