Stephen and a few friends created these together on a weekend in the Catskills. It's actually such a great idea for a little party, even if you don't have a woodland retreat of your own; invite friends to bring their own lidded jars, collect cuttings from your garden (or even from a hike through a local park), and provide supplies for everyone to assemble their terrariums.
According to Stephen, all you'll need is a bit of charcoal (available at nurseries), which helps with drainage and absorption of odors, then some potting soil, some moss, and any plants and other found items you want to include. He suggests making a 1/4-to-1/2-inch layer of the charcoal, then a layer of soil on top.
You can add a bit of water at first to moisten the soil, but afterwards water only very rarely, because an enclosed terrarium creates its own internal ecosystem. Just keep your terrarium out of direct sunlight so it doesn't dry out. Stephen says he gives his own terrariums about a 1/2 cup of water every couple of months (and the amount you actually need will depend on the size of your jars and how dry it is in your home); he also recommends some occasional "editing," to replace plants that aren't thriving or remove plants that are getting too big and taking over the space.
To see more of what Stephen is up to, check out his lovely blog.
(Images: Stephen Orr)