Draw a light guideline for where the mouth sits on the face
Sketch a simple line for the upper lip curve
Add the lower lip curve beneath it
Draw the cupid’s bow on the upper lip (two small curves meeting in the center)
Indicate the corners of the mouth with small marks or short curves
Add the philtrum groove above the upper lip with a short, gentle curve
Refine the lip thickness by tracing slightly thicker curves for the upper and lower lips
Outline the mouth opening shape with a curved line for the lip opening
Add a subtle inner line to show the separation between lips (especially on the lower lip)
Shade the area under the lower lip with a soft gradient
Shade the inside of the mouth opening darker than the lip edges
Add a highlight on the lower lip edge and/or the center of the lower lip
Adjust corners and overall curvature to match the expression (smile, neutral, frown)
Erase unnecessary construction lines and clean up the final contours