glass ornaments (mine were smaller than the regular sized ones)
little Christmas trees (found mine in Hobby Lobby Christmas section)
glitter (mine is clearish, but white would work... possibly even salt if you were out of glitter?)
Take the metal top out of the ornament. Sometimes there are little strands of glass blocking the way, so try to break those gently (and don't use your fingers unless you like pain).
Put a small dab of hot glue on the bottom of your tree.
Using the tweezers, quickly and carefully place your tree where you'd like it. If you don't get the tree exactly right, it's super easy to pluck it up and start over. When I went to make this, I couldn't find my tweezers, so I used the end of a bobby pin which didn't give me the length or ease of tweezers and I had trees sticking to the sides of ornaments, so I had to pull up the trees and replace them until I got it right.
Once your tree is in there, put a little glitter in the bottom until it covers the base of your tree. I used a post-it note as a funnel to get it in there and it worked well.
Put the top back on and admire your cute little ornament!