Practice, practice, practice
I'm interested in watercolor portraits, and I'm currently taking a different class on that topic, but unfortunately the portrait class I am taking is not great at explaining how to draw the lips, or the logic behind the shadows.
This class however was great at filling in that blank. At this stage I have gone through the class 2 times
*attempt #1

*attempt #2

* And attempt #3 using a photo reference to draw the lips structure.

Even though in the final version the chin is a bit small and the upper lip ended up a little bit asymmetrical overall I am very pleased with the results as this class also helped me plot the shadows much better.
For reference here is a drawing of the same lips before and after taking this course. The difference between the two is just 1 day.
BEFORE

AFTER

I'll be definitely looking at the class with t 3/4 view of the lips.