Step #1
Draw the top lip lightly. When someone smiles his or her lips stretch and get thin.

Step #2
Draw the bottom lip making it thicker than the top.

Step #3
Darken the corners of the mouth, making two small triangles and lightly sketch the bottom of the top teeth.

Step #4
Draw the points of the gum line that will separate the teeth. Draw the bottom indentions of the top teeth.

Step #5
Add a middle gray tone to the top and bottom lips. The top is usually darker.

Step #6
Tone in the gums and lightly add the bottom teeth.

Step #7
The teeth are never pure white. Carefully add layered tone to the top teeth, bottom teeth and use your eraser to pull reflected light from the bottom lip.

Step #8
I added a bit of tone to establish the facial muscles that pull the lips and some small high lights on the teeth the make them look wet.

Step #9
Use your eraser to pull out more sparkles on the gums, teeth and lips.

Step #10
Lastly, add sparkle to a few bottom teeth and light wrinkles in the bottom lip. Smile, you're done.
