I started with my thumbnail, a basic sketch using a blue pencil, here I make decisions about perspective and the general composition / layout before scanning it.
Once I am happy with the sketch I start to 'ink' the illustration and tidy up the line-work.
Then I add colour. I use 'flats' which means selecting an area of the illustration and filling with 'flat' colours (eg green and orange for the cars) I then start to airbrush and shade them to add definition.
Finishing the illustration involves building up the layers of colour, concentrating on each area in stages and building up the overall detail consistently. Finally I add a few effects to add some drama to the lighting.