You use special light-sensitive paper, overlay it with uniquely shaped objects (leaves, keys, office supplies) and leave it out in the sun to develop. You can also use film negatives or images transfered onto transparency material. This paper-and-sun process is a specific example of a very old photographic printing process called cyanotype.
The other day I came across a modern version of this printing process, Lumi, a Kickstarter project that is about to wrap up. It offers 3 different colors of light-sensitive dye for transferring photographic images onto natural fabrics. It even comes with an app to process your photos for the best results. Hurry if you are interested in getting a kit, their deadline is almost up (they are already WAY funded)!
I just picked up some sun-sensitive paper and fabric on sale at Urban Outfitters (and am probably going to get on this Kickstarter)--I'll post the results if anything turns out decent!
[image credits: Make Grow Gather, Artful Adventures, Mom Culture, Photojojo, Arts 'n' Craft Supplies, Photojojo, Mess-cipes, Lumi Kickstarter (x2), How About Orange]