I got the curtains from a second hand store called ReSales close to Stachus (Munich poeple will know where that is). And the idea of filing the curtains came from here.
Since I work in an office and we just threw out a bunch of old ringbinders (and I hate throwing away stuff that's still usable) this was my new project:

I needed 4 ringbinder clips to attach to one window. Sprayed them white first.
Then pink.
I cut out some cardboard and painted it pink to glue to the back of the clip for a better grip on the window, since I couldn't really screw it on.
And then clip clip clip clip... filed curtains!