Dark paper works best, such as black construction paper. Laminated paper will last for multiple years, and even newspaper will provide a funny effect for a night or two.
It isn’t hard to sketch out basic shapes on large paper, but feel free to search for easier ways. Searching for Halloween stencils yields many images that would work well on a window (I borrowed several from this article by Jeffrey Rudell on the Make: blog).
Printing a black stencil will use up expensive printer ink quickly, so don’t do it! It’s more efficient to cut a big, dark design out of big, dark paper. To save printer ink, I turned my dark silhouettes into simple outlines using the open-source graphics software GIMP (Edge Detection and Threshold are your friends).
To print on a large scale, you may want to use a program like PosteRazor to enlarge and print out your outline over multiple sheets of paper.
Once I had the outlines, I taped them to my roll of heavy paper and used a razor to slice both the stencil and paper at once. Pizza boxes make excellent backings for slicing paper with a razor blade or X-Acto knife.
Finally, tape the silhouettes onto the window and turn on the lights. The effect is dramatic from inside and outside the house.