Parts: I used gnome, deer and mushroom cupcake toppers from this etsy shop. The glass containers were found at Ikea. (I tried to thrift some, but came up empty handed.) The soil, rocks and baby tears were found at my local Lowe's. Activated charcoal - or carbon - was available at the pet store.
Assembly: First layer, at the bottom, is pebbles for drainage. Next comes the charcoal (for enclosed terrariums to filter water and air). Thirdly, spaghum moss keeps the soil from slipping. Now your soil, and finally, your plants. This site has a wonderful plant list, but I found most items were out of season. We settled on a flat of baby tears.
Care: The baby tears have already grown and are easy to pinch back. The terrariums are pretty much self-sustaining. They require only a little watering every 2-4 weeks. If there is too much condensation, just pop open the lid. When the soil starts to look dry or cracked, add some water. That's it!
For a wonderfully visual step-by-step tutorial on terrariums, you'll want to visit Pretty Little Things.