This is my workflow when drawing stuff.
- I draw them on paper, using a pen.
- I scan them as greyscale, and normally 300 dpi.
- Then I open them with GIMP, try to use eraser to clean the drawing a bit.
- Increase Contrast till all dirts are gone.
- Increase Levels to make the lines stronger and solid.
- Change image mode to RGB.
- Add another layer, with white background, and set it's mode to multiply.
- Start colouring on the newly added laters, and add more layers if required.
- And that's it.