How did I do it?
I took one of the sheets of plastic and drew lots of circles on it, then coloured it in with permanent markers. It took aaaaaages as I needed to let it dry after doing a small area, or it would smudge. (I ended up doing the colouring-in in between rounds of playing computer games!) Finally after a couple of weeks, it was all coloured in. I then measured the window, and cut enough rectangles to range across the window with gaps in between. I took a piece of ribbon I had lying around in the hobby room, and stapled the rectangles to it. I started in the middle and stapled rectangles alternately on each side until the ribbon was full. Then I tied a simple knot in each end to form a loop and hung it from the nails that keep the window-screen in place.
Here's a less artistic, but more detailed shot:
I have plenty more sheets of plastic stashed away, and I'm wondering if I can whip up a lightshade for the hobby room. With a simpler pattern this time, I think. ;)