It's basically just window film, that reflects sunlight and creates a rainbow on a wall, on the floor, window sill, everywhere else, depending on how large is that piece of film you put on your window.
What you will need:
- window film: make sure it reflects sunlight so it creates rainbow;
- marker and ruler.
Then just stick that piece/s on your window, no glue needed, it should stick on its own. :)
Enjoy your own indoor rainbow!