It's important that you take the time to walk through and complete the Understanding 3D Form tutorial before you're ready to take this next step. Anyone who thinks they are beyond the basics presented there is kidding themselves, myself included. No matter how good you may be, if you dont get the basics it will come back to bite you.
Step 1. This is the biggy. This step dictates the entire head drawing, and or pose for that matter if you are attaching an entire body to this. Start with a sphere, freehand will do, so long as it reads as a ball. You are going to divide the sphere into halves, then fourths. There will be 8 sections total. Think of this as a 3D sphere, which you should do anyway, because what you draw will ultimately be 3 dimensional. Once the sphere has been evenly divided you are going to find the golden cross section that dictates and determines the size, shape, tilt, character, and proportions of the head to be drawn. The cross section determines where the brow line begins, and divides our face plane in half with a centerline. These divisions also help determine where the ear will sit on the side of the head. Amazing how it all comes down to a simple cross section, isnt it?
The centerline of the face is derived from our cross section. This centerline is drawn next. Note that the centerline of the head doesnt follow the form of the sphere once we have established the cross section. Why? Because, the chin extends away from the sphere shape. If our chin was a part of this sphere, we would have a pretty darn round face. It doesnt have to be any particular length at this point, it is only there to establish the two main divisions of the head, or divides the head into an even left and right half.
With a minimal degree of shifting of one feature or another, all faces are fairly symmetrical. I say minimal degree of shifting of features, meaning not everyones eyes are perfectly even across the face laterally, not everyones ears are laterally even, etc. Lyle Lovett for example, really asymmetrical. But his face still is evenly divided, and it has all the components required to call it a head.
Once you know the tilt, have found the cross section, and laid in the centerline of the head, your next step is to shave to two side of our sphere on either side of the cross section. This will help you begin forming the side planes of the head. Why shave them? Well, the skull isnt actually round like a ball, but flattened on all sides with a fairly round top for the skullcap. Keeping those round side planes would actually be deceiving, and the skull drawn would end up a little too wide, awkward looking, and the ears would be mounted on these little tiny ends, with no real cohesive attachment to the whole
Step 2-Now that you have the cross section established, the next thing you will do is divide the head into thirds. This is the golden division you will use to lay all of the feature elements of the skull upon, i.e., the eyes, nose lips, ears, and so on The primary, or stock look for the average human is even divisions between these thirds. But once understood, these divisions will be your guide to drawing charactures, character types for stories, or whatever. These divisions are as important as the golden cross section that began all this. The upper third is what you start with. This establishes the divisional units of the skull, i.e. it creates the volume to which the other 2/3rds will be measured. The upper 1/3rd is from the hairline to the brow line.
The 2nd division establishes the location for the nose, ears, and eyes. Remember, when you find the nose, you have also found the ears. They follow the top of the nose, or brow ridge, and the bottom of the nose division evenly with just about everyone. Knowing this will help you. I see many drawings and paintings where the ears look like they were pasted on to the skull after the fact because of little or no understanding of proper proportion, or placement were privy. The middle divisions go between the top of the brow, to the base, or bottom of the nose.
The lower 3rd sets up where your mouth goes, and finishes the skull with locating the bottom of the chin. Thus, this lower 3rd goes from the bottom of the nose to the bottom of the chin. These divisions of thirds wrap entirely around the skull. These divisions remain evenly proportioned no matter what the tilt of the head is. So if the head is looking straight up, all the divisions will be shrunk down to little segments, but still even in their division. This is soooo important in placing all the features on the head.
The side division of the head helps place the ear in its correct position, to the left of the vertical division line for the right ear, and to the right of the vert line on the left side of the head. This vertical division also tells you where the jaw begins, since it attaches to the skull just in front of the ears.
When it is all said and done, and this golden division of thirds is well understood, and heavily practiced to the point of intuitive understanding, the rules of proper thirds can be broken. This is where character types are created. They dont necessarily follow proper spacing and placing, but rather take on their own divisions. This is valuable information for spacing and placing the features on the face. I cant stress this stuff enough. I see many a drawing and painting that just didnt quite work because these basic principals werent known. Remember it well, and you will always draw a properly divided, and proportioned head, or head attached to a body in a figure drawing.