What I always do is start out with a simple shape. In this case I'm going to draw a mouth from the front. Let's imagine our character is smiling so I'm going add sort of a curve line that indicates where that smile is going to go and then this oval describes the shape of the lips. I think of the top lip as sort of two triangles with a little indentation in the top. I think of the bottom lip, I tend to, again, just map out a sort of a little square shape to describe the bottom lip and then bring up the sides like that. The bottom lip tends to be a little bit thicker than the top lip.
And again, now that I've got those basic shapes, I'm just going to land the actual lines. One thing that really helps the mouth look realistic is when you get this little shape, I don't know why, but top lips always do this funny little thing in the middle. If you look at a mouth really closely, there's always kind of a little edge to the bottom. Again, now I'm just adding my shading lines, they sort of, they describe the light hitting the mouth but they also describe the actual shape of it.
So now we can do the same mouth with a more neutral expression. When the mouth smiles, the lips sort of tend to get a little thinner so if I drew this exact same mouth in a more neutral expression the lips would appear a tiny bit fuller. When this mouth is smiling, it stretches the mouth sideways and that makes the mouth a little thinner in the center and it makes the sides pull a little straighter.
So lets imagine this character with the mouth open. Again, I tend to think of everything as squares when I'm blocking things out. So if I look at a person smiling I tend to see their teeth as a literally just as a white box and that helps me sort of figure out the construction of their mouth. And again, when the character is smiling it's going to stretch those lips out so this little bump in the center of the top lip is going to flatten out when they smile.
It's good when you can, when you're dealing with something like teeth for instance, where it's a lot of little complicated shapes, if you can just think of the teeth as sort of a single white square shape. That really helps you to get into it. Then you can add a few little details around the edges to make it look more like real teeth and just a little indication of gums. You'll find if you try to draw every little tooth, it kind of makes the person look kind of ghoulish. So I tend to do just a few nicks around the edges that give me the appearance of teeth.
Now I will show you more of a side view and how to construct that. You always want to remember that the top lip goes a little forward of the bottom lip so it's sort of a triangle on its side. That'll be the mouth. And that top lip, the little bumpy part in the middle is just going to overlap that bottom lip kind of like that. The bottom lip again, is a little bit fuller, it's a little bit more circular in shape. The top lip looks a little more triangular from the front, from the side I mean. The best thing to do is just draw constantly, and just draw everything you see and you will build up a bank in your memory of images that you know you how to draw without having to constantly reference everything.