STEP 1.First, start with the top lip, which I've broken into two steps. You want to be careful not to make it curve too much here, but you also don't want it to be straight.
STEP 3.Now add in the bottom lip. You want it to run parallel with the top lip and keep it's ends about the same distance away from the top lip.
STEP 4.Now finish it off by adding the teeth, with yet another slight curve. And you don't want the teeth to be drawn at the far side of the mouth. Remember lips stick out from the teeth a little, so draw the teeth a little before the end of the lips.
STEP 5.And don't forget the extra little curve under the bottom lip, because the bottom lip has a definite thickness to it the people expect to see. This concludes my tutorial, let me know if it helps at all.
source : http://www.dragoart.com