Exercise 1: Draw a mouth in the following three steps
Step 1: start with the slightly wavy line between the lips. Step 2: draw the shape of the upper lip softly as a shadow shape, without a thick edge. Draw a line and/or a little shadow under the lower lip (feel the hollow there with yourself). Step 3: With some dashes/hatching you show the curve of the lips extra well. You can make the line between the lips a little darker in some places, such as the corners of the mouth.
One has thin lips, the other full lips, one a narrow mouth, the other a broad one. In the face, the mouth is always slightly wider than the distance between the eyes.
Exercise 2: Try to draw these three different mouths. Start each time with the line between the lips.
From the side you draw the mouth again like a wave. The line between the upper and lower lip is different for everyone and in every portrait. You don’t need thick edges for the lips. The colored mouths come from old Italian portraits.
Exercise 3: Redraw these mouths from the side or find other examples in drawn or painted portraits.
Exercise 4: Draw some silly mouths, such as with the figure ‘Smart dude’ or someone with a very big mouth (a bladder jaw) in front of someone who’s a bit calmer.
What you learn a lot from is how to copy mouths (or other parts) in existing drawings. You can also copy them from photos (image) but that sometimes has drawbacks: photos often have spots of shine and shadows that can be confusing. In addition, smiling mouths with bared teeth are very difficult and suit a photo taken quickly much better than a slowly drawn portrait. In drawn or painted portraits the mouth is usually closed because the model has been sitting still for a quite some time. A subtle smile is of course possible.