First, find a photo of yourself of someone and it helps to have a lot of contrast. An area of high lighting, a mid-tone and some dark shadows makes the work go easier. The image I used here was tough when it came to darks and mid-tones. I did this quickly because it was a teaching example and I had some difficulty which was awkward. Here's why:
Its really easy to get back down to zero. This is Denril (other artists like Mary Borgman use frosted Mylar but Denril is easier to find) and denril absorbs graphite readily but a firm stroke of the eraser wipes it away ENTIRELY! Its like the "undo" button in Gimp.
I will write more later, but for now, the general idea is this: Build your way up slowly. Map your highlights in with very light masses of gray. (I recommend against pencil, it looks wonky) Build towards your darks. Use stumps because the natural oil of your hands makes for total black. This oil property has me thinking of using walnut oil (food grade and less toxic than linseed but more expensive) to set up my darkest darks. I look forward to making time to make that happen. So, its really hard to lighten an area. Its super easy to go from super dark to zero but not so easy to take a dark tone and soften it to a mid-tone. That can be done with a cloth but I found it was harder to do that than to plan ahead.
One last thing for now, I used a Grid and copied. I let students who were nervous blow up their image and trace for the layout process, then copy for lights and darks. I will try to dig up a few student examples from my hard drive.