I scan in the drawing, and then lighten it up and give it a tint in photoshop. I then print it out on watercolour paper, first to work out the colours, and then to paint. For some reason I cannot retain the info needed to set up the printer the correct way to print out right the FIRST time, so I end up with duplicates, which is VERY handy when you have a 5-year old who wants to paint too!
I layer on the paint - using Gouache, and then use ink to finish it. Here is the final art!