The best 2 methods are 3m double-sided tape (not the foam variety) and trim clean the back of the jewels well and then place them on one side leaving the opposite side covered so you can trim to the edge with a quality pair of scissors or an X-acto blade... OR drop $15 on a can of Super77 spray adhesive, allow to cure for a bit, clean the frame off with alcohol and then stick them on. They don't call it super for nothing.
Also, you'll want to use a fine pair of tweezers with the Super77 so you don't smear any glue on the front side of the jewel with your fingers. It can be a B to take off. LOL