Now you may all be asking yourselves, "Where's the image? Why don't you post it? I can't see the head!" Well, I will post it at the end, if you really want it but I should warn that it's not perfectly lined up for front and side viewing. Also, since we're concerened with just the mouth area, showing the entire head may distract you from your goal, which isn't what I want. In this image, you can see that I select an edge and extruded it along the path of the lips, altering the shape of the polys to match the image as much as I could. Sometimes all you have to go with is a front or side view,this time I drew both so we're lucky. In situations where you don't have both, use a mirror or feel your lips to get a sense of depth that may be lacking. You may be surprised what exploring yourself this way may teach you about faces.