The mouth is important for the facial expressions.
In the sketches below the mouth is drawn from a frontal viewpoint.
In addition to that, observe the round shape of the lips. For a realistic outcome I recommend to use soft round strokes instead of straight lines.
Do not be afraid to draw teeth. In the sketch below, I started with the dark areas, the interdental space and the opening of the mouth. The shape of the teeth can be drawn with a light pencil. In the end you can apply the darker shadows to achieve a more realistic outcome.
Female lips are usually fuller than male lips. There is also a difference between natural and lips with make-up. The latter are usually a bit darker (see right sketch).
Below you can see the mouth from a lateral and an oblique heightened position.
Instead of describing more anatomical details I advice to observe the shape of the lips. Look at the shadows and highlights, and then try to draw what you see.