This are my tools
First of all. If you never draw a mouth, the best thing to begin is this. Sometimes learning how to draw human parts in comic or anime version can help us a lot.
Start with cross. It's no necessary to do it with a ruler, but if you are no very confident about your skills, this can help a lot. This cross has 10 cm wide and 5 cm hide.
Draw to lines to narrow the width of the mouth. I have draw them to 4 cm from the vertical line. This is just the sketch, then you can modify the width and tall of each lip.
This is the geometric scheme, don't worry, is not necessary be perfect. In the human body perfection is just a myth. Help: the cross will be helpful to draw this lines.
The best way to shape the upper lip is with circles. Draw it softly because you are going to erase it. Draw the circles as shown in the picture. The contours will serve to draw the angled profile.
The line that divides the lips it's the same when you draw an open book. It looks easy, but that line can give you some problems. Don't worry, it's just a line, I know you can do it.
Draw the lower lip with rounded strokes.
Now let's shape, join the lips at each end. The lower lip end before the top lip.
Okay! Now we can erase the circles and the lines! Time to model the lip
This part is where you imagination and taste goes on. I like thick lips, and shorter too. So this is the time where you start to gave shape to the mouth as you please.
Let's talk about the light and the shadows. This is important at the time to give the shape. First we have to chose where the light come from, draw the shadows following the shape of each lips.
The upper lip is darker than the bottom one because the light is not direct. The brightness is in the lower lip, at the right. Remember, the light come from the up to the right.
Dark the edges of the upper and lower lip. Dark the grooves and reliefs of the lip from the top (following the shape of the lip)
Mark the line of the lower lip with circular strokes at the ends.
Let's search volume in the lip. Star at the ends with dark strokes and degrading it.
Make a slight sketch of the central well. Ranging from the upper lip to the "nose". It's just a visual reference. Make the shadows and highlight the bright spot to accentuate the wet nature of the lip
Mark the line between the lips, and other places of the lips if you feel that they are too soft
Softly, with the pencil make some lines following the shape of each lips. And that's it! You finish it!
- Pencil sharpener