How to Draw Mouths

Figure Drawing and Composition, Richard G. Hatton

The form of the upper lip, shown in A is caused by the shadow line of the turn of the lips ahown at B. There is a hollow depression at C which folds into the mouth. At D the lower lip end. The mounds below the lip at E are reflected in the two sides of the chin at F. The chin's prominence at G is reflected by the tip of the nose.
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As with the head is is useful to have a knowledge of the skeletal structure.

how to draw mouths
Art Drawing Made Easy, Campana's Popular Library

The Nose and Mouth from below, in three-quarter view and from the side
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Buchanan's Initiatory Drawing Lessons
For the Use of Writing Academies and Private Families
Edinburgh, 1828