Step 1. Start by drawing the top middle section of the upper lip, in the diagram you can see the blue lines describe how this portion is curving out towards the left because we are drawing these lips facing slightly to the left. 

Step 2. Draw two steep drops for the side portions of the lips, this is how we create the opening of the mouth. If we drew the lips with the normal decline we would end up drawing the mouth closed.

Step 3. Add in the bottom portion of the lips, notice how they curve slightly towards the left, the right side is longer and the left side is shorter (or steeper).

Step 4. Now we will draw in the teeth, you do this by drawing a horizontal line slightly below the upper lip and another horizontal line slightly above inside of the mouth.

Step 5. Add the tongue by drawing a curved line that starts by the teeth and ends near the beginning of the bottom lip (on the right side). Notice how it looks similar to a sunset disappearing behind the horizon line.

Step 6. Darken in the inner space of the mouth.

Step 7. Add details such as shading lines and a circle to suggest there is a highlight caused by the lips being slightly wet.

I hope this was useful to you! Make sure to grab the worksheets and print them out, reading drawing instructions is much harder than practicing from examples.