You need to find black butcher paper. I got mine at a local party supply store. You will need enough for all your desired windows that you want to cover, but it can be pieced together as well.
With an exacto knife, cut out any shapes you like
You can trace it first with a white crayon, but I prefer to just free hand it. You can also look at a pumpkin carving book for ideas that won't punch out all the way. Then put it up in your window, covering the entire area with the paper.
This photo shows the effect during the day, as the light comes through.
And at night, a peek from outside, on my porch. Cutout door and dollar store Frankenstein with spooky streamers.
Now, I was able to snag some old creepy movie posters that finished off my scary windows. Video stores are not allowed to re-sell these, so if you know someone who works at a video store you may be able to sweet talk your way to a poster of your own.
Movie posters are designed to hang on a lightbox, so the image is reversed from the other side and light can filter through.