To learn how to draw a mouth, we start with a sketch.
Start by drawing a heart shape for the middle portion of the upper lip.
Then draw the lines for the upper and lower lips.
Now that you know how to make a mouth’s sketch, make two marks and draw a line in between.
Mark the middle and add two other marks above and below.
Draw the upper center.
The top of the heart shape is on the upper mark.
Now that it’s not a sketch anymore, the heart shape is much rounder.
Join the two sides, like so.
The lines are slightly bent as they come to both sides.
Draw the middle of the mouth.
Draw the lower part beginning with the center.
Now we already have a more concrete mouth shape.
Then add the lines inside the lips.
Darken little by little to correctly create contrast and depth.
You need to draw a shadow on the lower lip, right under the middle line, when the mouth is closed.