Line the lips& Create a base

Create the shape of the lips you want,
use a darker colour to give the illusion of depth.
Fill in the lips, focus on the corners of the mouth and use small
short strokes from outline to the center of the lips.
Then use a lipstick/lip tint start in the center of of the mouth
and use short blending strokes.
Be careful not to go too close to the outline of lips
(you don't want to erase the depth you created),
but use this step to make sure everything is blended.