Start with sketching the outlines. Draw this very lightly so you can erase if you make any mistakes. This step doesn't have to be complicated as long as you can see the lower and upper lip. The upper lip is usually smaller and has the cupids bow while the lower lip is wider. Add some lines to the lips to indicate the creases.
Start adding some shading to the lips. Try to keep the area in the middle more white (especially on the lower lip). This is where the reflection will be.
Because of the scribbles, the lips probably look a bit messy. Blend the graphite and don't worry about the creases that are disappearing. You can always redraw them.
Keep repeating the last two steps. 1. add a layer of graphite. 2. blend the graphite. Do this until you're satisfied with the darkness. Add more layers to certain areas if it should be darker.
What can help to make the texture more clear is to sketch the layers in the same direction as the creases.
This is where we're going to add details. Use a pencil to darken the creases. Don't draw them too dark and make some of the creases lighter than the other. Use an eraser (preferably with a sharp corner/point so it's easier to make details) and add highlights.
Add some shading to the skin area around the mouth. There's usually a bit of shadow under the lower lip
Darken a bit more of the line between the upper and lower lip as well. There's also often the "philtrum" which are those two vertical lines between the nose and lips.
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