How to Draw Mouths on Your Fashion Figures
Draw lips in profile view.
Draw a sideways triangle that points to the right. Draw a dashed horizontal line that cuts the triangle in half.
Add two curves at the left side of the triangle, making the top curve narrower than the bottom.
Erase the left side of the triangle and trace over the dashed line with a solid line to separate the lips.
You can also draw another look in which the upper lip is slightly more forward than the lower lip, reflecting a realistic view of how the front teeth go just in front of the lower teeth.