purpose:
To determine where values change when light moves over an uneven surface by using a range of values to paint the different values in a realistic, monochromatic self-portrait;
To effectively communicate an image that looks like you through careful examination of the values and structure of your face.
The skills you need to paint facial features are good attention to detail. If you focus on what you are doing, use a small brush and take your time you can make every detail look amazing. What you need to do for adding value is look at your picture and number the drawing you are drawing over. You need to plan so you know what values you need to put where.