Hey there! First of all, I'd recommend understanding the volume of realistic lips before trying to simplify them. Having a good grasp of their three-dimensional form will make it much easier to simplify them while still maintaining a convincing sense of volume with just a few lines. For this type of stylized lips, one thing that really helps create volume is using the "T-lines." It's a small detail, but it makes a big difference. :) Also, notice how the front plane of the lips (I'm not sure what it's called either) overlaps the sides. Hope this helps! | Artwod Feedback