Credit to Flickr user LübeckerJung for the photo. Unfortunately, he gives no description. So, if you have any instructions on how to do this and where to get the materials to do it, post it in the comments for all those interested.

UPDATE: Thanks to commenter adsfds, who I’m assuming just randomly hit his/her keyboard when entering his/her name, for pointing out that these decals are sold on Etsy.com. Check out a larger photo below.