From the middle, nearer to the bottom, click and drag in a curve to make a smile:
Repeat the action a little lower down to finish the lips:
Remember, you can undo (Ctrl + Z) until the lips look the way you want them to.
Because of the symmetry setting, anything done on the left is repeated on the right, reducing work and making it easy for you to make a pleasing shape.
Because of dynamic topology, the rough polygons of the original shape are smoothed out so that all the details of the drawn shapes show through. This effect is easier to see if the view is switched to wireframe mode (Z).
In the Tool Box, increase the Radius setting...