Start with a light horizontal guideline where the mustache will sit
Mark the center point under the nose
Sketch the left half with a curved line extending outward from the center
Mirror the shape on the right side
Add thickness by drawing a second line below the first on both sides
Round or taper the outer ends depending on the style you want
Connect the inner edges near the center with short curved lines
Refine the outline to make both sides balanced
Erase any extra guidelines
Shade or fill in the mustache as desired