I would suggest beginning by quickly blocking out all the major regions of the face:




that took me about 40 minutes to do, and it is far from perfect, i have a bad habit of making everyone look older than they are in real life and it's much harder for me to do on the computer, but it is sufficient for this first step. 40 minutes may sound like a long time, but it is well worth it to get all of the rough placements and forms set up.
NOW, once you have DONE ALL THAT can you BEGIN to refine the image. Dithing, adding colors, smoothing, tight drawing, these all should come well after you have a base image to work from.
Try it, youll find that preparing your images versus not preparing them, the results will be 100% better 100% of the time you prepare them.