First of all! If you want an in depth understanding of drawing and portrait painting. I have written a descriptive and thorough book thats available on:
First I start to measure the position of the middle of the mouth/where the 2 lips meet. It’s the area where you see a visible dark horizontal line of shadow. The measure could as an example be from where the chin ends, the center of the pupils or the lower part of the nose. It’s just important that you have a primary fixed point to measure from.
The center of the mouth could just be marked as a simple and soft line to start with.
By always using the same or a primary line or fix point I will minimize errors. Quite often the corners of the mouth are in line with the pupils. So if you start marking where the eyes should be on the portrait that area could be the main measuring point.
As of now you should have stated the horizontal central line fix point of the mouth. This is the shadow that becomes visible where the upper and lower lip meet.
What I do from here is to just follow up on that fix point and draw the line and its form and shape to each corner.
As you can see on the photo a line is not just linear. It goes up and down until it meets the corners.
You can also make a line that defines the upper lips top. Occasionally I make a soft line that defines the bottom of the lower lip. As the lips curl out on top of the teeth and they have an oval shape you will notice that the lips are exposed least to light near the central horizontal line. This makes the shadow more dominant. As the lips turn out more on top of the teeth they will become lighter as they are more exposed to light.
From here I gently sketch all values starting from darkest to lightest. It may sound simple. But if you see the hole area as values and you start methodically every time. The subject will materialize very slowly and surely.
On the example above I have used a 0,3 mm pencil to sketch the hole mouth. Depending on the value. I simply press harder or sketch more layers on top of each other – which is called cross hatching
If you want to see how I draw noses click Here