Draw a simple horizontal guideline for the mouth placement
Mark the center line of the lips
Sketch the upper lip shape with a soft curve
Sketch the lower lip shape with a larger curve
Connect the corners of the mouth lightly
Refine the lip outline with smoother lines
Add a slight dip in the upper lip center
Add volume to the lower lip
Draw the inner mouth line if the mouth is open
Add teeth only if needed, keeping them subtle
Shade under the upper lip
Shade the lower lip lightly
Add highlights on the lips
Erase construction lines
Clean up and darken final outlines