Start with a template. Draw around this onto two pieces of black A4 sugar paper (construction paper) I secured it with paper clips to make things easier . This design was tricky to cut out but older children could do it with some help and patience. If you have a small group I would be tempted to do this beforehand with a craft knife.
Cover window with tissue paper. Cut this into strips beforehand. Glue the other A4 black paper to the back. Hang in a window.
Go to this post for a PDF of the template that you can print.