For stamping on tiles the trick is to make sure they don't have a shiny smooth surface, or that they don't have a coating on them...these were actually very "chalky" when I bought them, which is normal. You just need to rinse them and let them dry before you attempt to make them beautiful.
I did two different versions both using the Medallion Stamp that I love so much, but the possibilities are endless! If you'd like to give it a go, I'm having a class on Saturday March 26th at 2:00 PM. Visit my website for the details!