I started out with drawing a sketch on a plain piece of paper, I used myself as a reference. I thereafter scanned the drawing in, with a HP Deskjet Ink Advantage K209 printer. I opened Painttool SAI and cleaned the sketch up with a Bamboo Pen & Touch tablet, by tracing the lines that I had sketched with a Pen, size 1. I coloured everything in with the Marker tool (Which I do not recommend, since you don't draw at full opacity which can cause quite a lot of frustrations when finishing your drawing) and used Gimp to add the little details, like the text on her shirt.
Quite a simple process, right?