A mouth can be drawn in three simple steps. The most important is the line that defines the upper lip and thus defines the smile, frown, or whatever you decide to draw.
Go back to the doodles and pick a mouth you want to draw. Sketch it.
Once you have your mouth outlined, draw two curvilinear lines. The top lip will be convex compared to the mouth you drew, and the bottom lip will be concave. The top lip will have a little dip at the center.