There are always so many approaches to art about as many as there are artists. We all have our favorite ways to bringing ourselves into a piece of artwork and have a tendency to always fall back on that favorite way to begin. The best thing any of us can do is to realize to begin with, then remember there are different ways to thinking about things. Don’t bottle yourself up in a routine or habit and not bother to study and look at a different way of working.
A good for instance here would be when beginning a drawing, painting, or sculpture of a head. As I said, there are many ways of approach. I have seen some artists paint the background around a loose sketch of the head/bust, choosing to deal with the detail after the background is complete, or nearly complete. This is fine and if there is a lot of hair that is going to overlap the background this is probably wise. After the background is finished though you still have to deal with the face, neck and bust. Depending on your experience and skill you could just begin by adding shadows and darker areas of the head. Personally, I work from shadows up finding it easier to get things located in the proper places.
The head and face are a myriad of planes. If your skill level is as such you don’t have to think about these planes anymore you can jump into the form of the face because you know where everything should be located. As someone who is just beginning it might be wise to actually do many sketches either from life or photo to study these planes. The face certainly not being a flat plane makes it necessary to know the angle of these planes, location and size. Of course, we have to consider here everyone is different. There is no formula that is 100% accurate for every face. True, we know the mouth length usually lines up with the pupils of the eyes, the inside point of the eye lines up with the edge of the nose. But, as I said, these are generalities and will be different from person to person. This is what defines that person’s features.
While working on the human face is it absolutely necessary to keep looking at either model or photo, referencing these features unique to that person. If you are making up the individual from imagination it won’t make any difference.
It goes without saying it is also advisable to have a working understanding of the bone and muscle structure of the face and head. This is necessary to paint, sculpt, or draw a believable, realistic portrait.
To have mastery of the human face takes lots of practice and keep observance, especially if you are matching an individual. Don’t be afraid to practice before you take on that portrait commission!