All the other features are triangulated back to the centerline and keyed off of the distance between the eyes. Most of us do not have perfectly symmetrical features so remember that when you are establishing the triangulation between the reference points that the only fixed point will be the one on the centerline, the other two points will more than likely vary in elevation and distance from each other and the centerline.
Lines on the face divide volumes from one another, natural lines are plane breaks from the skeletal and muscular/fatty masses under the skin. Lines from age express similar mass divisions on the face and occur perpendicular to muscle forms. Where these lines occur are usually divisions for forward protruding masses, or sideways protruding masses. Along with the line break there should also be a value change of some sort. A hint, if the light source is powerful enough to diminish this subtle lighting of form chance, especially on a forward-facing portrait, the other indicator is temperature change, which is a fancy way of saying color change.
I also feel it is important to know how to cartoon or line gesture as much as sight measure or whatever other academic tools the artist trains to use. Without this skill I feel that another aspect of likeness, caricature, will not translate very well and the likeness can easily border line on not really looking quite like the individual or have a measured out look to it rather than it "feeling" right in its effortlessness. These are short gesture sketches, each about 3 minutes each attempting to exaggerate the shape language and become familiar with the design of his portrait.
This was roughly 30 minutes using a Blackwing Soft pencil and 11 x 17" xerox paper, cheap but effective. When I am sketching to learn I do not hold dearly to any of the drawings, they are all subject to abuse and radical change. When I get nice paper I feel guilty when I have to butcher up a drawing to correct it, and I feel like I should not have made a major mistake on such important and expensive paper, which in the end is a goofy way to think. It takes sacrifice to learn the tools of our trade.