How to draw portraits isn't just about achieving a likeness its also about getting the character, the personality and vitality of the sitter. We put marks down onto the paper and the mark it has left has made a statement, whether the marks make any sense or not is another matter but it is our own personal statement that we have made. The technique that you use with your pencil is also important in catching the essence of the sitter.
The first principle you have to think about in how to draw portraits is the structure of the face, without structure your portrait will fall down like a chair with no legs. The second most important principle is form, the way the forehead recedes at the sides, the way the ear is formed with little curves and crevices, without this form your portrait wont make any sense and will be a mish mash drawing. The third principle in drawing is technique, that is the way you use your pencil to express
how you want to draw the portrait.