So here's roughly the steps I took when making my Halloween post past the sketch-phase! At first I wasn't very happy with the linework, but when I tried coming up with different ideas I felt I was losing the spirit of the original sketch. So I kept it as accurate as I could.
The hands were a little weird. I decided to paint them in without any linework as I felt the original ones were too detailed. In the end, I think think the "floating" hands looked pretty cool.
The lighting was annoying. I'm not really sure why, but I was overthinking it a lot, focusing on the green highlight more than I should've. Everything came together when I focused on the head. I basically painted it dark blue, similar to the background, then used the airbrush tool across the bottom to make the green gradient. I added the highlights last-- keeping in mind where the green light was coming from (the bottom of the jar), and finally had something that looked nice :)
I decided to do the same thing with the rest of the character-- and kept the shadows / highlights on separate layers to make it easier to adjust. After that, most of the other details fell into place.
Hope that was a bit insightful! Feel free to ask questions if you have any.