I started by wrapping the base in polymer clay so the resin wouldn't leak out of the bottom. Since the base is made of metal it's fine to bake it with the polymer clay. If you use a base made from plastic, use air dry clay instead.
After the bases finished baking, and cooled off add air dry clay and glue to secure the trees in place. I then added paint to the bottom of some. I added resin mixed with white ink, to look like icy water, to the bottom of the bases.
I topped them off with fake snow from Hobby Lobby. You can find the fake snow and most of the other supplies in the miniature scenery/diorama/model train section of hobby lobby.
I glue the bases closed after adding the snow, then glued bezels on top. This will allow them to be attached to chain, string, ribbon, etc. And they can be made into ornaments, jewelry or other crafts.
Miniature Glass Domes: http://www.etsy.com/search/supplies?q=miniature+glass+dome&view_type=gallery&ship_to=US
Miniature Diorama Trees: Hobby Lobby
Miniature Trees with snow: Hobby Lobby
Fake Snow: Hobby Lobby
Resin: Micheal's or Hobby Lobby
Watch the video here: