1) Sketch out the basic design to work from. here I have done it straight onto photoshop but you can do it on paper and scan in if easier
2) Duplicate layer and increase the size by 150%. Airbrush in basic shading using soft brush tool, black, 50% opacity
3) Create new blank layer and make it 75% opaque to see the base layer. Choose a colour for windows and spray in. add black at the top for depth. Use eraser tool to rub out any 'overspray' on all the levels
4) Merge last 2 layers and then create another blank one. Using a fine brush for outlines and a thicker spray to fill the window reflections and the ground