Learning how to draw faces using values is a good way if you are a beginner and must train your eyes. Fill up the sketchbook wth quick value sketches from black and white photos. Not worrying about anything else but the values. Value is the first thing that your eye absorbs. By developing sharp eyes sensitive to seeing the values, it will help you draw well with accuracy. Don't think about what the subject matter is, Don't look at it as an eye, or nose or mouth. Just see the patches, pixels of values. Start anywhere you want without any lay-ins or landmarks. and go from there. Pay attention to light and darks. You can scribble, stipple, however you want, but don't worry about making it clean.
This portrait sketch demo shows pencil drawing techniques using mostly the values. It is done from a black and white photo of the actress Scarlett Johansson. The artist starts with a random spot, and chooses somewhere in the eye, then he evaporated from there, comparing the value differences as he goes. Constantly observing the lights and darks.
The artist continues to scribble away in this fashion all the say to the finish, where he slows down a bit to tighten things up and render more.
This practice is very good fro developing your painting skills as well. There are many approaches in drawing pencil portraits but comparing values in one of the most efficient and easier way because values are what makes things realistic.
When drawing with values, sometimes it helps to squint with your eyes to see the large groups of values, but it is also important to keep in mind that a squinted eye sees things a bit darker than they really are. So it is good to squint just to see how things are grouped but it is not wise to copy down the actual values.
Although this is one good way to learn to draw faces, It is also important to learn how to draw faces using only the lines and with the knowledge of the forms .