Every mark you make with pen and ink is absolutely 100% permanent. If you are attempting a portrait in pen and ink, you are either impressively confident in your ability to perform a complex task without error, or you are anticipating errors and in fact are wondering where they will lead you. Remember as you proceed that the act of drawing itself is beautiful. Enjoy making lines and watching your creation unfold in front of you.
Preparation and Beginning Stages
Gather your materials. Since human faces are organic in nature and have generally delicate features, the best tool tool to use is a pen with a fine-tip nib, or even a crow quill pen. For a quality piece, use acid-free paper. Experiment with the pen before beginning your drawing, even if the nib has been used before.
You can begin your drawing one of two ways: either by drawing a light sketch of the subject with a pencil and eraser before beginning your pen and ink study, or by diving in with pen and ink straight away. By far the second option is the braver of the two, and more likely to render interesting distortions and abstractions. If you choose to begin with a pencil sketch, draw lightly enough that the pen and ink will cover it up; whatever the pen and ink does not cover up, you can erase.
Start somewhere that makes you comfortable, whether that is with the outline of the face, one of the eyes or somewhere else. Presumably a pen and ink portrait is not your first time drawing a person, so you may already have a method for drawing people that works for you.
Shading and Textures
If you've used pen and ink before, you're aware that shadows are often represented by a series of lines crisscrossed over each other. The more dense the nest of lines, the darker the shadow. To draw the features of a face, you'll want the shadows to begin and end gradually, to reflect the soft contours of the face itself. Start by making the shadows lighter than you anticipate you'll want them to be, and build them up deliberately. Step back from your portrait constantly, reflecting on your work.
When you reach textured areas of the body like the hair, use the pen and ink to your advantage. Draw every hair. Follow the lines of the hair from end to end. Remember, this is a slow medium that requires patience. Don't start to scribble when you get bored -- step away. Come back when you're ready to continue. In the end, you will be rewarded with the satisfaction of a rich, complex drawing.
For dramatic effect, deepen the darkest shadows in the portrait a shade or two beyond their natural appearance. Increasing the contrast between the dark and light will add interest to the portrait.
Don't fret over any mistakes -- especially mistakes that can't be hidden. Accept them, even accentuate them.
Some areas -- like the teeth and ears -- are best left under-addressed. Run the teeth together. Don't draw each individual tooth. The results will distract attention from areas of the face that will be a more emotional draw to the viewer, such as the eyes.