Step 1: The Outline
You want to start off by making a dot somewhere on your canvas or paper that is the focal point. Draw a bunch of boxes (rectangles of squares or whatever, and then draw lines from the corners of the boxes to the focal point to create a 3D building kind of look.
Step 2: Fill In
Now to create the image on top of your buildings. Only paint on top of the buildings and create a different color on the sides.