Forget everything I've ever told you about gemmaking. Go see HCC Cosplay Group's Polyester Resin Jewel-Making Tutorial instead. They've pretty much got it pegged.
All I would have to say in addition to what they've written is that, when gluing foil backings to your gemstones, use a clear glue like Clear Gel Tacky Glue. (Sometimes, when you glue foil to your gem, it creates an airtight seal. White glues won't dry completely and they'll make your gemstone look cloudy.) I'd also recommend when gluing to just put the barest amount of glue onto the back of your gem and spread it carefully with your finger, giving the back of the jewel just the barest covering of glue. You should then press the gemstone onto a smooth piece of foil and carefully cut the jewel free when it dries hours later.
If you have any questions about anything, just e-mail me and let me know.