The key to drawing a mouth is to notice where the light and shadows fall. The upper lip is usually in shadow.
First, measure using your “Perfect Eye”: where is it placed on your face? How wide is it?Make as many angle lines and EYE measurements as you can. Your mouth is about 2 EYEs long. How many EYEs below your nose is the line between your upper and lower lips?
Once you have set your lip sizes, you begin with value. Make sure you notice the shadows under the lower lip as well.