trial and error. and lots of saving. GIMP is a great program, but there are so many options in it there's no way to tell you a, b and c. Use Free select to choose the sections you want to work on. Copy the selected areas and post them on a new layer. Use the colour dialogue box to experiment with the colours that are there. Beyond that its down to what you want to do. I'm not really that experienced with digibashing.
you should be able to easily adapt this for your needs, as it's pretty much the same across the board: http://www.tfw2005.com/boards/tutorials-how-tos/214529-paint-net-digibashing-tuturial.html
