did this in photoshop, i'm using Painter 9 more than photoshop nowadays, but i basically do it the same way, but each time its a little modified.
step 1- measure out the face, sketch in where everything is supposed to be. dont be concerned with details, because none of this will be seen in the painting..drawing bfore u paint is just measuring. measure the distance between the eyes, the height and width of the face, the angles of the face. the more time you spend doing this the more accurate it will be

step 2- pick two hues. one light and one dark. i chose funky colors on this, if you want it to look more life like, choose more brown flesh tones, but make sure theres a noticeable difference. this is easy, since you have everything measured out, you look at the shapes of the light areas and shadow areas of the face..when the light and shadows blend into each other, pick a spot and make it a solid line. make the whole thing two values, looking at the abstract shapes

step 3- blend them together where they need it, and add some midtones in

repeat till it looks as close as you want it to be. this can go on forever, the trick is knowing when to stop

step 4- i make a new layer and get out the brush tool with around 2% opacity. then i put warm colors, reds yellows oranges in the light areas, and blues purples and greens in the shadow areas. warm colors pull things forward, while cool colors push them back. after that, i just add the highlights
finished product, time span was between 1 and 2 hours
