1. Initial sketch: here I very very quickly sketch out a loose idea of what I want the composition to look like. Usually I do a small thumbnail and if I like it, I scale it up so I can work out the details.
2. Rough Re-working: Here I roughly sketch in the details such as background, proportions and positioning of the figure.
3. Finished Sketch: I go in and further refine the sketch and make sure everythings in place so I can lower the opacity and on another layer start working on the lineart.
4. Lineart: self explanatory for the most part!
5. Flat Colours: just fill in the natural, simple colours for the figure/background.
6. Mood: Here I make an overlay layer and create the lightsource, and a couple different multiply layers to create the mood.
7. Shading: I make a multiply layer and go in and shade everything according to the light source, then do highlights with an overlay layer. After I have the cellshading down, I user the 'marker' tool in SAI and add gradient to the shading for more diversity in tone.
VOILA. After this, I took it into photoshop and fooled around with the colour balance as well as lowered the opacity of all the lines. If you want more detail on anything in particular, let me knowww!