Choose your subject. Although you should not hold yourself to low expectations, you should assume that you will draw a flawed image because you are a human and you can't be perfect. There are two methods you can use to counteract this problem.
Some faces are so well known that they are like icons--trying to draw them accurately may seem nearly impossible, because we have been exposed to their image so often that nearly any reproduction of their face will seem amateur. You may find that you feel less pressure to produce a recognizable figure if you choose someone less well-known.
Alternatively, if you choose the face of a celebrity with an obvious and distinctive feature (Marilyn Monroe's beauty mark or Bob Marley's dreads), that feature will work to your advantage. The distinctive feature, whatever it is, serves as a gateway to your work, leading the viewer to believe that they see the person you have drawn.
Begin drawing wherever you feel comfortable, but begin with outlines only--no shading.
If you are drawing the face only, you may wish to start at one eye and use that eye as a reference point for the location of the other features. For example, if you start with the left eye, you know that the right eye will be straight across the face. Pretend there is an invisible line (or draw a real line, very lightly) from the inside corner of the eye that you have already drawn to the inside corner of the eye you will draw. The distance between the two eyes will be approximately the width of one single eye.
The nose will be centered beneath the two eyes, on a vertical axis in the middle of the face. The mouth will fall on a horizontal axis beneath the nose.
In this way, using each new element of the face as a point of reference for the next element you place on the face, you should build the features of the face and body.
Step back from the drawing and give yourself some time away while you work on other projects. When you return to the drawing, you should have a clear perspective on whether or not you're on the right track.
Shade and add details. For features like the hair, do not make each individual hair with your pencil--treat the hair as a single entity. To create the effect of hair, pick out clumps of strands or individual strands to draw.
Take more time away from your drawing and return after you have had time to forget what it looks like. When you return, your mistakes should jump out instantly. Maybe an eye is not lined up perfectly, maybe the body is shortened or hands are too large. If you find yourself certain there is a mistake but not sure what it is, try turning the drawing and the photograph you are drawing from upside down. Hold them side by side and flick your eyes back and forth between the two. Where do you see variations? Where does your drawing differ from the photograph?
Make changes to the drawing while it is upside down. Turn the drawing right side up and study again. Make more changes as necessary.
Find someone to provide constructive criticism of your drawing. This should be someone that you trust to tell you the truth, and not someone who will shield you from their true thoughts for the sake of making you feel good about your work. Ideally, this will be someone with a background in art who can identify where your drawing is right and where it is wrong. People who have no skill in art may be able to recognize that the face has flaws but may have no way of identifying what those flaws are.
Present the drawing to this person without comment on the subject. It is important that this is the first time this person has seen your drawing--anyone who has watched the drawing in its various states of creation may have a skewed or biased perspective. If your chosen critic recognizes the subject of your drawing without prompting from you, you have at least partially succeeded. Ask your critic how you can improve the drawing and where you think the drawing may be incorrect.
Thank your critic and accept the input graciously.
Use what you have learned from your critic to make improvements to your drawing.