Summarize this article:

Grab the eye of an eye pin with pliers and carefully heat the metal over the flame of a candle or gas stove. Only heat the metal for about 5 seconds.  Work carefully to prevent accidental burns. Use an eye pin that is just a little shorter than the width of the resin piece. Grab the resin piece with one hand and carefully poke the straight side of the heated eye pin inside.  Press the pin in until it extends roughly halfway through the resin piece. This will only work if the resin is only partially cured. If you allowed it to cure completely, it will be too hard and unyielding. Once the metal eye pin is cool enough to touch with your bare fingers, slide a small jump ring into the eye. This completes the project and turns your resin piece into a wearable pendant or charm.

Summary:
Heat a metal eye pin. Poke the metal into the resin charm. Attach a jump ring to the loop.