How to draw realistic lips

The most important thing to remember about lips is that they are all unique. The worst mistake you can make is to assume that if you can draw one pair you can draw them all.

Tip: when first learning to draw anything, draw by sight, not memory. If you learn to draw something first by memory, you will get in the habit of making the same mistakes over and over again.

Lips are mostly drawn by shadows. We think they're a solid line, but remember, they're just a more sensitive continuation of your skin.

I suggest beginning by drawing the center line since this is the darkest part of your lips. It will not, and should not, be perfect.

I like to continue by shading in the shadow projected on the bottom lip by the upper lip since these are some of the darkest values.

Follow the values around the bottom of the lip. The lips are much more complex than we might assume, but remember to copy values, not shapes, and it makes it easier.

Follow by shading the top lip. This part is a little more difficult (at least to me) because the values are generally more soft and subtle.

And, that's basically your finished product (I threw in some visual context for ya).

I'm sorry I couldn't go more into detail. The main thing is to remember that you are tracing values, not shapes. The only thing that separates your lips from the rest of your face is it's shade.