NOTE:  These steps are written using Illustrator, so before you begin, in the View menu, choose Grid, then Snap to Grid. (It can also be done with pencil and paper, and an ellipse template or if you're good at drawing ellipses and understand ellipse degrees, freehand them.)

1.  Using the Pencil Tool, draw an "egg" to represent the head, and stretch it to fit vertically in a number of grid squares that are divisible by 8. This is important because you're going to need to divide your drawing vertically in half, then that half in half, then that half in half using horizontal lines. The final division will be in 8ths of the total height. You'll see what I mean soon.

2.  Measure halfway down from the top of the "egg" head to the chin and, using the Pen Tool and Shift key to keep it horizontal, click the beginning point and endpoint of a horizontal line (this is line 1, the "eye line") . Positioning it accurately will be easy using Snap to Grid.

3.  Measure halfway down from  line 1 to the chin and draw another horizontal line 
(this is line 2, the "tip of nose" line).

4.  Measure halfway down from line 2 to the chin and draw another horizontal line 
(this is line 3, the "mouth line").

5.  Draw another horizontal line at the bottom tip of the chin (this is line 4, the "chin" line, since we're counting). 

In the drawing above, I placed ellipses on the level of each horizontal line. Note the near side of the ellipse (which is really a circle rotated away from you to lie at an approximate 15 degree tilt). In the flat 2-dimensional drawing, as you can see, the bottom of the ellipse line sits on the horizontal line. 

Depending on the angle of view, ellipses would be drawn at different degrees. If you were to look at the head straight on (not from above or below, but at eye level) the ellipses would simply appear as a straight line. (In that case, the circle is completely laid back, flat, so all you see is the edge.. or a line). If you were to look straight down on the head from above it, the "ellipse" would be a full circle. Any view between eye level and looking straight down on the top of the head determines the ellipse degree. The higher your view from eye level, the higher the ellipse degree. As you see in the drawing on this page, the view angle is nearly at eye-level. It's approximately 15 degrees above eye level, thus the approximate 15 degree ellipse used in the drawing.

I'd already done a freehand drawing of the head in Illustrator using the Pencil Tool and drawn the horizontal lines using the Pen Tool, then positioned them as described above, using a Grid and Snap to Grid. Here's how I created the ellipses:

1.  Used the Ellipse Tool to draw one ellipse.

2.  Used the Selection Tool (and sometimes the Shift key to constrain proportion) to stretch the ellipse to the right size and degree, then positioned it on one of the horizontal lines.

3.  Clicked the ellipse with the Selection Tool then, using the Scissors Tool, clicked on the nodes at each side of the ellipse, to cut and separate the back of the ellipse from the front (the top half from the bottom half, as seen in the flat, 2-dimensional drawing). 

4.Clicked the back of the ellipse (top half) with the Selection Tool and in the Stroke palette, set the Weight at 0.5 pt, clicked the Dashed Line box and set all of the dash(es) at 
0p6 (0 picas, 6 points) and all of the gap(s) at 0p2 (0 picas, 2 points).

5.  Clicked the front of the ellipse (the bottom half) and set the Weight at 1 pt.

6.  Clicked both halves of the ellipse and in the Objects menu, chose Group. 

7.  With the ellipse still selected, held down the Alt key and dragged to copy it three times. 
Now I had four ellipses. 

8.  I positioned each of the ellipses on the horizontal lines then selected all of them and, 
using the Transform palette, clicked on the Horizontal Align Center icon to center-align all 
of the ellipses.

9.Now, with the horizontal lines and ellipses in place, I used the Direct Selection Tool to adjust the indentation for eyes, and placement of nose, mouth, overall head height, top of head, chin, and ears (not necessarily in that order, as I moved around the drawing, adjusting and readjusting until it looked about as "perfect" as it was going to get). 

10. To see how this drawing looks turned to the right, make a selection of the drawing image and (depending on your software terminology) do something akin to "Flip Horizontal". 

• In Illustrator, go to the Objects menu and choose Transform, Reflect, then click Vertical and Angle: 90 degrees. Click the Preview button to see how it will look, then click the OK button.
• In Photoshop, go to the Edit menu and choose Transform, then Flip Horizontal.
• In Painter, go to the Effects menu and choose Orientation, then Flip Horizontal.

12. Save the new image with a different file name so you can refer to it later. Now you have two views of the head, one facing left and one facing right.

When I have some time, I may do more drawings of the head similar to this one, but with varying head positions, or views.

I hope this helps someone, and we can thank a couple of people on an e-mail list for stirring up a discussion on drawing the human face and head and reminding me of the formula shown above. Without it, my attempt at drawing a proportionally correct human head would not have been as good (good?). (There's always room for improvement, and this was good practice for me.)