Some paint practice. It’s pretty easy to draw a mouth when you don’t have to worry about trying to fit it on a face.
This was less of a focus on construction and more on trying to understand the texture. It looks too “wet”, which would be ok if I was drawing someone with some gloss or lipstick, but I wasn’t. Strong highlights are a really easy way to give believability to a texture, and it makes everything pop out for illustrative purposes. However, overuse of them can make surfaces all have the same sheen, which is a common habit for a lot of people.
The only real way to get better at painting is to study materials and textures.