How to draw a mouth.
The first thing to observe are the "contour lines" drawn over the mouth. The red lines convey a dimensional illustration of the construction of the lips.
Notice how the shadows dictate that the upper lip be rendered darker. In contrast, the lower lip protrudes, which makes it lighter. The corners of the mouth have more shadow. In most cases, a good trick is to make the corners of the mouth very dark.
The blue areas in the illustration above show a shadowed area of the upper lip. As a general rule, all of the upper lip is darker in comparison to the lower lip, but the blue shadows that you see here are even darker.
Almost 100% of the time, you can observe 5 different sections when you look at someone's lips. In drawing these sections, you will simply emphasize them a little, or a lot.
Drawing the bottom lip:
The highlighted area in orange (above) shows an area of the lower lip that sometimes is a little darker, or gets a little more shadow. This is where the lower lip curves inward. The closer to the edges of the mouth, the darker (or, in shadow) the bottom lip gets. But, remember - the bottom lip is usually lighter than the top lip because it is facing upward and gets more light cast upon it.
The highlighted area in green shows shows a shadowed area around the mouth that should not be forgotten. This shading indicates the structure of the face around the mouth. The mouth isn't just plopped on the face, it is a three-dimensional thing, with surrounding muscles and structure. Usually, there will be some shading (subtle, but there) at under the lower lip, and around the corners of the mouth.
The area highlighted in white all around the lips is another often overlooked detail in portrait art. If you look closely at someone's lips, you will see that we all have a bit of a ridge around our lips.
When someone has a 5-O'clock shadow, it'll be particularly noticable. In most cases, whiskers won't grow on this particular area around the mouth.
* Sketch the outline of the lips.
* "Block in" the shading of the lips, putting more shading on the top lip, and leaving a highlighted area in the middle of the bottom lip, and some highlighted areas on the upper rim of the top lip.
* Add more shading and rendering. Don't forget the "ridge" around the lips, and the shading around the mouth that suggests its structure. Add the darkest accents of tone to the corners of the mouth