Start with a light horizontal guideline for the mouth placement
Mark the center line between the upper and lower lips
Sketch the basic shape of the upper lip
Sketch the basic shape of the lower lip
Keep the upper lip thinner than the lower lip
Define the cupid’s bow on the upper lip
Add the corners of the mouth
Draw the philtrum above the upper lip
Add the shadow under the lower lip
Shape the lips with smooth, natural curves
Refine the outline with small asymmetries
Add the line where the lips meet
Shade the upper lip darker than the lower lip
Blend the shading softly
Add highlights on the moist parts of the lips
Darken the corners and the inner mouth
Draw subtle texture lines on the lips
Adjust the shape to match the face angle
Erase construction lines
Finalize the edges and shading details