When dry, Royal Icing creates a cookie topping of hard sugar candy. When Royal Icing is piped into a shape and allow to dry by itself, it creates sugar candy that can be pasted onto a cookie, cupcake, cake or other frosted bakery. The benefit to not piping shapes directly on cookies is if you make a mistake, you do not waste the cookie.
Refer to an article I wrote entitled Create Royal Icing Designs Without Cookies for more information and search the Internet using phrases like “Royal Icing Transfers” or “how to use Royal Icing transfer material.”
These two videos by Julia Usher provide detailed information about how to make simple and complex transfers. She recommends .003 (3 mm) flat, clear acetate sheets which are available in pads at art supply stores, rather than parchment or wax paper. It comes in 9×12 and 14×17 pads.