You can build a terrarium in almost any glass container, small or large. It's your own perfect little world in a jar.
First, fill the bottom with some rocks, followed by broken up charcoal, and potting soil.
Next, find some moss or low-growing plant. (We found ours in the back yard.) And add a little friend, if you have one on hand.
Upkeep: We spritz ours with a water bottle every so often (once a week or two). I also take the top off every once in awhile to breathe a little. I've read that little water droplets on the glass is okay, but murky fog is not. Also, if it starts to smell bad, definitely give it some air.