First, we can lower the brightness and contrast both of the sketch so we have greys and whites to work with. Already you can see that you've made some nice metallic-ish effects here and there. Let the accidents happen.
The Smudge tool works just like finger-painting. We push the white areas closer to the lines so it gets 'cleaner'. In the reverse, you can smudge a darker area into a lighter one for shadow effects.
For final touches, you can use black color set to low opacity so it takes several strokes to make a harsh black line. (what i mean is im not sure and im scared so ill use a light black over and over on an area til im sure it should be there, like watercolor).
Highlights that tell you where the light source is can be put on later when you're sure where you want the light to come from too. I'm never sure myself. Usually I'll dab the whitest 'ink' on places where I think the subject's 'highest' areas are if it was a real thing.
Note the 'Opacity' of the brush is set rather low at 29%. The non-committal types won't find a problem with this.
There are many many, many really good free paint programs out there that can do these effects or better. Try The DogWaffle Project http://www.thebest3d.com/dogwaffle/free/
if it still exists cuz it was kickass and FREE
I think 'smudge' is called 'finger-paint' there.
Simple is best so try to get used to and master very few tools instead of a whole lot of confusing ones. This whole process used only 2 tools: the smudge tool, and a normal round flat Brush (using diff size and opacity, black or white). Best to treat it like it's on paper
Same in real life, use few tools that you can use well. I end up using a mixed media of ordinary cheap ballpens, Stabilo's, correction fluid if I had this on actual paper. Guerilla Art.