There are many ways to draw a portrait, but I wanted to put some information and links here that would give a basic introduction to portrait drawing. Some people have questions how to start? They may be overwhelmed by the whole idea and not know a method or set of steps by which they might break down the process of drawing. We can easily recognize the face of a subject and we may know instinctively what is a good drawing, but somewhere in the middle of drawing it is very easy to get hung up and lose track of the whole picture and focus on some non-essential detail. This can make a sort of puzzle in wondering what path we might have missed, or what signs along the way we did not notice.
Portrait above by Lon Brauer. Note that a portrait can be rendered very tightly, or very loosely, but if things are placed with regard to the form, the image will feel right nevertheless. It doesn't take a lot of detail to produce a convincing portrait.
Below is a video by a Lon Haverly. In it he does not get too fancy with his finish, but he follows a certain path in which he sets up a basic shape of the head. He says that you must "keep the form in mind." He defines the shadow side of the head and then proceeds to lay in the eyebrows halfway down the head. Following that, he lays in the bottom of the nose and then defines the basic form of the nose with a shadow side. You cannot place an eye until you have a face to put it on. You also cannot place a face until you have a head to place the face on. Therefore, you must work from large ideas to more refined details.
Some things to watch out for are whether the whole head shows the same light source, whether the shadows and light areas agree, and whether you can feel the sense of the form in our mind, a sculptural dimensional sense of the thing. If you intend to draw a cartoon with no shading with pure outlines, do that. If you want to draw a realistic portrait, then draw the form accurately with shadows. Decide one way or the other and then stick with it. There is no halfway.
Below is a portrait that makes an attempt at shading, but the whole thing looks flat because the shadows do not agree and not enough attention was paid in advance to the architecture of the face that the features must rest upon. There are no major dark and light relationships to lead the eye through the image and to establish an important focus area.
Q: How might you improve the portrait above?
Why do I say that you must render form or avoid it completely? It is because shadows placed without attention will not add definition but will possibly add confusion. The head may not stand out against the background. The depth of emotion in the eyes may not be properly conveyed. The form of the torso may not be evident.
Avoid thinking of a person as a sort of Mr. Potato Head, in which all the features must merely be placed in the appropriate place, and either outlined, or scribbled in their positon casually. Humans don't have swappable parts. We are instead organic machines that function together as a whole. There is a rhythm internal and external that unites the forms as one. I hope this information is at all useful to beginners.