i make these!!! lol (i'm not that etsy'er but i do make them.)
you need bottle caps, resin of your choice, and pieces or pictures to put inside. you also want to be able to either solder a ring on the top or screw a whole into the top, so be proficient in either small drilling or soldering. the resin you can buy in micheal's or pearl. its literally a bottle of resin and a bottle or catalyst that you mix to the right specifications. crazy stuff that resin. i put photos in there, and i also cut up an old tarot deck i had that had neat little pictures and did some layering stuff... wicked fun. and wicked rewarding when you're done cos people are like, "how'd you do that?" lol