Find a harry Potter related image on the internet
Once you've found your image, make sure it fits the diameter of the stone and if you happen to have a square image make sure to trace the stone around it and then cut out the image.
Use craft glue to glue the image on the stone. It will dry clear so don;t worry, Make sure you spread the glue around the image and or on the back of the stone.
Then grab your hot glue gun and your ring base and glue it on the back of the image that is now glued on to the stone. Press hard to make sure it stays and add a little around the edges of the image to make sure it does not separate from the stone.
Leave it to dry for a while and there you have it.