When I make bottle caps I print out on Kodak gloosy photo paper. Then let the ink dry. Then I punch out the image and glue it on the cap. Then let it dry. Then I spray the image with a sealant and let it dry. Then I use 3D lacquer to finish it off. Then let it dry for at least 24 hours.
Here are some directions http://www.inchbyinchdesigns.com/IBIDbottlecaps.html