I have chosen the beautiful lips of Jessica Alba to do a tutorial on.
Unfortunately I do not have a larger version of this photo to give you. However, larger versions of the actual drawing is available, just click on the smaller ones.

As usually I start drawing up the shape/lines of the lips, to make sure everything is placed correctly.

I start shading the upper lip with the charcoal pencil, rather roughly but without pressing too hard.

Keep shading the lips with your charcoal pencil, while fading out some of the shading with your finger once in a while.

I continue doing this and by now I have added the darker lines at the bottom of the upper lip, which makes the lip starting to take the round shape.

Upper lip is darkened a lot by now. Keep in mind not to make the shading too hard at the center of the lips, as we are going to add some highlights soon, with the kneaded eraser. Look at the reference to make sure where the lightest parts will appear.

I have shaded the area over the upper lip by now as well as added the hightlights above the lips. Adding the hightlights over the lip will help determend the fullness of the lips and add to the shape.

The main part of the upper lip is done and I start paying attention to the under lip. Use the same technique as the upper lip, but keep in mind the lower lip will be slightly lighter and have more highlights. While shading the lower lip do it softly and try to avoid the middle part as we will have lots of highlight there.

The lips are almost done by now. Add the final finish by adding a few very dark lines in the darkest areas, such as the bottom of the upper lip, the corners of the lips as well as under her lower lip.
Use your kneaded eraser to make some thin lines. The final highlights and darkness is what gives the lips the final touch and shape.