Uh... more specific? But I'll describe my technique.
Just FYI, this is under GIMP.
First, I copied the sketch onto a new, second layer. Then, used the Color Selection tool to pick all of the bad parts of the sketch and deleted them. I cleaned it up a bit, and what was left was a line drawing on an otherwise-transparent layer.
Next, a new layer under that. There I just colored it in, using the appropriate shading.
After that, I applied bloom (too lengthy a description, but I can explain it if you want me to) to both the figure and the bad background.
And that's it.