Lips come in all shapes, sizes and colours. In my opinion, the fundamental skills to learn is perspective, lighting and shading... almost anything can look good with great lighting and shading ;)
Perspective is not really an issue in this portrait, so it comes down to lighting and shading. The main principal at work here is just building up the image, working and re-working until you are happy with the final piece. Don’t get discouraged in the initial stages when it looks rough, it only matters what it looks like when you’ve finished :)
There is no ‘right’ or ‘wrong’ way to go about it. Some would say you need to sketch first, or use basic shapes, some don’t. Have a look at other artists techniques and adopt one or adapt to suit yourself.