Drawing lips is an art exercise in understanding the most complicated feature of the face because the mouth itself is a feature of continual change.
Unlike the eyes, the nose, and the ear, the mouth will have the most drastic changes based on the emotions you would like to portray in your character.
Due to the various positions we draw a mouth, there is an endless array of things to consider that extends beyond what you can learn in one sitting.
Thus, for this particular lesson, we will only deal with the basic structure of the mouth so you can have a feel of this facial feature.
The lips can be divided into three different sections: underneath the nose, the upper lip, and the lower lip.
When you're looking underneath the nose, you will see a groove in the center followed by two planes on either side. There is a definite crease that separates the section underneath the nose and the beginnings of the upper lip.
One thing to learn how to draw lips properly is to understand the planes. You have discovered this when drawing the nose. It's the same thing when you draw the mouth.

There are three shapes possible underneath the nose. The most important shape is the groove found in the middle just before it gets connected to the mouth.
To either side is a flat plane that connects to the groove. This creates the lip bridge. Use your digital brush to create these drawing planes. Again, just draw it in digitally.
The end result should be a nice plane to both sides and a groove in the middle. Make sure the planes are separated.
The next step in drawing the mouth is the learning the upper lip. There are many ways to draw the upper lip as each fold will determine the character's facial appeal.
Usually, thin upper lips results in masculine type characters while thicker upper lips results in feminine type characters. It really is up to you and how far you want to expand on this.
Luckily, the upper lips have the same general planes just with different variations. It is these variations that create different style lips.

There are three planes of the upper lip when you draw a mouth. The central plane protrudes out and at an angle while the sides taper off.
Try to draw the same shapes in your digital art canvas.
As well, see if you can continue those angles. Again, upper lips on humans will have the same planes regardless of what type of characters you are planning.
Of course, to draw a mouth in its complete state, you need the lower lip. Most of us make the mistake of assuming that it's just an oval lining completing it. However, that is not true.
While the areas underneath the nose and the upper lips are crisp creases, the lower lip is not.
This is because the lower lip contains two bulges on either side that most of us don't really notice. But trust me, it's there!

You will draw these two bulges on either side. If you can, segment these two bulges in a circular manner so you can get a feel for the overall shape of the bulges.
If everything is done correctly, you will draw a mouth in its most basic form. Obviously, it's not done yet. You need to blend things together so it looks like a real mouth instead of this 2d outline.
We are almost done! This step involves blending in your lip shapes together using the smudge tool.
From there, use a chalk brush and just simply redraw some lines here and there to make the lips realistic.
There are noticeable spots of white near the lower lips as you want to show some form of lighting reflecting off it. Other than that, draw simple lines that show curvature of the lips and to define of of the skin lines there.

When you are done, you should have something that resembles the lips.
Regardless, what you have done in this lesson is to get yourself set up for how you will paint the mouth in your digital art application.
All in all, this lesson on drawing lips should get you to realize that having the basic shapes are crucial to getting the right foundation for whatever you are drawing.
For now, keep practicing the basic shapes. Give it a bit of modification so you can draw a mouth correctly to simulate other different mouth variations.
I hope you enjoyed this lesson on how to draw lips. Please make sure to submit your artworks!
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