Lips are the hardest body part to draw
Tutorial: How to Draw Lips - 3/4 View http://rapidfireart.com/2016/10/07/how-to-draw-lips-from-the-34-view/
How to draw lips
Tutorial: How to Draw Lips A very simple way to draw lips. You can even use this method to draw different types of lips by making just a few changes in step 1.
I like how all the lips are different and in different colours but they're all so accurate
Teaching the students the basics of how to draw, shade, illustrate, etc. the features of the human face. Once they learn these basics, they can use this information as they draw their own faces.
How to draw realistic mouth, teeth, and lips
Tutorial is interesting, but I just love the style - messy but still neat. Seems free.