You can begin and carry through your painting entirely with body color, or you can begin it with frottées, and paint solidly into that. Take these two methods as types, and work in one or the other, according to what are the special qualities you want your work to have.
If you have never painted a head, and have some knowledge of the use of paint and of drawing, I would suggest that you make a few studies of the head and shoulders, life size, in solid color, and on a not too large canvas, say sixteen by twenty inches. This will leave you no extra space, and you can devote your whole attention to the study of the head, with only a few inches of background around it. You will probably make the head too large. A head looks larger than it really is, especially when you are putting it on canvas. If you measure them you will find that few heads will be longer than nine inches from the top of the hair to the bottom of the chin. Take this as the regular size in drawing it on your canvas, and make the other proportions according to that.
Make a drawing of the outlines in straight lines, which shall give only the main proportions of the head, neck, and shoulders. Within this, block out the features largely. Don't draw the eyes, but only the shape of the orbit; nor the nostril, but only the mass of light and shade of the nose.
Construction.—In these studies avoid trying to get anything more than what will be suggested by this simple drawing. Use body color. Don't think of anything but what you have to represent. Never mind how the paint goes on, nor what colors you use, except that it is right in value, and as near the color as you can get. Put it on with the full brush, and try to get first the large masses and planes. Get it light where it is light, and dark where it is dark, and have contrast enough to give some relief. Don't try for any problems. Set your model in a simple, strong light and go ahead.
No details, no eyes, only the great structural masses. Try to feel the skull under these planes of light and dark. Have the edges of them pronounced and firm.
Do a lot of these studies; learn structure first. You will never be able to put an eye in its place in the orbit till you can make the plane of dark which expresses the bony structure of the orbit. You will feel the edge of the brow, of the cheekbone, and where the light falls on the temple and on the side of the nose. Inside of this is the dark of the cavity, broken for your purpose only by the light on the upper lid. Lay these in. Do the same with the other planes, and put your brush down firmly where you want the color, with no consideration but the simplest and most direct expression of value and color.
Now, when you can lay in a head in this way, so that you can express the likeness with nothing but these dozen or so of simple planes, you have got some idea of what are the main things which give character to a head. You will begin to understand how it should "construct." Into this you can put all the detail you want, and if the detail is in value with this beginning it will keep its proper relation to the whole.
Always when painting a head solidly, work this way. Get the action and character of the head as a whole. Block in the planes of the face and the features; and then go ahead to give the details which express the lesser characteristics. But always get the character, even the first look of resemblance, with this blocking in. Details and features will not give you the likeness, to say nothing of the character, if you have not gotten the character first by the representation of those proportions which mean the structure which underlies all the accidental positions of the detail of feature.
The Frottée.—If you want to be more exact with your drawing before you begin to paint, lay in your canvas with a light-and-shade drawing in charcoal. Then make a frottée in one color, and paint into and over that, as was described in the Chapter on "Still Life."
By careful and studious use of these two methods of work you can learn the main principles of painting portraits, and modify the handling as you have need; for all the various methods of manipulation are modifications of one or the other, or combinations of both of these fundamentally different ways of working.