I went to a local nursery and found some different types of live moss. When you call to ask if they carry moss, specify "live" or they will say, "Sure, we have Spanish moss and sphagnum moss--not what you want! The types I found were Irish Moss and Scottish Moss. I also found a different kind growing on the north side of the barn, so I put a little of it in the terrarium, too. If you dig your own, make sure to look at the soil on the roots to check for bugs.
In the bottom of your container, place some rocks for drainage. I found some river rocks and some small black stones. You can layer these if you like. It looks great to see the layers from the outside of the jar. On top of that put some good potting soil or mix.
Then, add your different mosses. I broke the moss apart somewhat to make different sizes and shapes. Mixing different colors makes it more interesting, too. After I planted the mosses, I added some of the more interesting rocks between the mosses to make it like a garden or mossy hillside. You could get really creative here and add miniature decorative garden items (mushrooms, garden tools, baskets, birds, etc.) After you get it just the way you want it, then water it and enjoy! If you cover the container, you won't need to water it as often.