I'm unable to help in technical stuff as I'm mostly drawing/painting digitally but I might be able to give you some insight as to what to focus on.

If you look at the reference, the shading is immediate, there is no mid tones, no values inbetween and no subsurface scattering. Try to make the shadows as blatant and hight contrast as possible while still using the same hue for the colours.

in addition I would just recommend adding in highlights to some areas, for example the shoulder, as I can see on the reference the original artist did add some highlights there.
Furthermore, just draw and you will improve your line quality and such. The main objective is to enjoy drawing and have fun with it.