Here you can see a video of him working. You can quite clear see how he works.
Step 1: Clear,cut,tear and stick
Cover the whole window, for the size you need, with one layer.
Step 2: To draw or not to draw
Step 3: "Cutfinger" he's the man with the cutting touch.....
Have fun making art and please show it here when you made one. I'll use this Instructable in my classes.