Include a link to, or a full copy of the Free Art License in your work, and something like the following:
Copyright © [the date] [name of the author or artist] (if appropriate, specify the names of the previous authors or artists)
Copyleft: this work of art is free, you can redistribute it and/or modify it according to terms of the Free Art license. You will find a specimen of this license on the site Copyleft Attitude
http://artlibre.org as well as on other sites.