I just learned how to make my own terrarium, and I'm so taken with the outcome, I want to teach you all how to make them to...
Here's my little drawing, which you can refer to as you read through my step-by-step instructions:
1.) First things first... Pick an awesome vessel for your terrarium. Ideally, you want something deep enough so that once you put in all of your layers, your little plants will only peek out of the top. Oh yeah-- and it has to be glass. It can be colored glass, but in order to really get the full terrarium effect, glass is a key component. : ) My "terrarium teacher" (friend Samantha) bought this phenomenal candy jar for one of her terrariums. You can find a similar candy jar here... She also recommended checking out the CB2.com stash, where she found all sorts of awesome shaped vases and bowls. I used a square glass vase that I already had, though I'll say, in hindsight, I could have aimed higher... (Ah-- retrospect.)
Bag of charcoal
Bag of potting soil
Bag of moss
Assorted small stones
Cute little plants to fill your terrarium. Be careful here. The temptation here is to massively over-buy because they're really cute. However, your terrarium will really look far better if it isn't jam-packed with plants, so channel a little restraint.
(Check out this Better Homes & Gardens article for lots of plant ideas and inspiration.)
3.) Okay! Now we're ready to get building our terrarium! Start by laying a thin layer (about 1/8 the total depth of the terrarium) across the base of your jar/bowl/vase. Then, drizzle a little water over a bit of the moss, and lay a similar thin layer of moss on top of the stones. (Don't forget to refer back to the drawing above if my descriptions are getting confusing...)
Once you've got these two layers down, start adding your soil. You'll want to add sufficient soil that it will be deep enough to plant your little plants, probably about 1-1/2" or 2" depending on the size of the plants. Now-- you guessed it-- you plant your little plants. Again-- you don't want to pack them in too tightly but you can also keep rearranging them until you're happy with your configuration.
4.) Once your plants are situated, you can add little decorative flourishes as you'd like. Extra stones, cool lava rocks, or neat little decorative tchotckas can really make your terrarium jump! Samantha had these terrific miniature houses that she added to one of her terrariums and they made an awesome addition! (You can see my final product above and Samantha's on the right.) As you may have gleaned, hers is far superior to mine. BUT! This is only my first attempt. Hopefully I will improve with time. : )