Start with two slanted lines. These lines will determine the width of the mouth. Connect the two lines with a smaller, curved line.
Add two ellipses right above the two slanted lines. Add one slightly larger ellipse under the whole thing. These ellipses will act as the outline to shape the upper and lower lips.
Connect the tops of the upper ellipses with another curved line, just about identical to the curved line drawn before. Using the shape of the ellipses, draw lines running along the top of each ellipse, ending at the end of the middle line. Do the same for the bottom lip (without the curved line).
Texture: To create the texture of the lips, you have to use lines that would run vertically along the surface of the lips. This, at the same time, creates the form. As you get closer to the ends of the lips, the lines will curve more.
Lighting: Always keep in mind the source of light; if the light is coming from the top, the bottom of each lip would be darker. The upper lip would also cast a slight shadow onto the lower lip. When adding in the texture and lighting of the lips, remember to keep some areas for highlights.
upper lip
lower lip
After adding the values and textures in the lips, you can also add the shadows around the lips.
Check out a related post on How to draw Generic Eyes.