Been thinking about doing one of these for a while. :>
NOTE: I use Paint Tool SAI
o1- Base lineart using the lineart layer. I use it on size 1 usually. Just go over your sketch basically.
o2- Flat colouring on a regular layer. I use the select tool to select the whole area I'll be working on and colour it with the pen tool.
o3- Add in a layer on top of the flat colouring, but under the lines. Set it to "overlay" and use a black airbrush just to darken a few places, blur it, then lower the opacity.
Add in a layer on top of this and set it to "luminosity", use a white airbrush to highlight a few places, blur it, then lower the opacity.
o4- Get a dark colour. Black, Darkblue, etc, shade with the pen tool on top of everything apart from the lineart. After this set the layer to "multiply" or "overlay" and blur it if you want.
o5- Add in a layer and airbrush across the hair in white, set it to "luminosity" and lower the opacity.
Add in a layer on top of this and draw in the hair shines with a small airbrush, set it to "luminosity" and lower the opacity.
o6- Add in the background on a bottom layer and add in an other effect.
Finished Picture >