I did a little researching, and this is my terrarium..
1. Sparkly clean glass bowl, jar, etc
2. pea gravel (I found mine in the aquarium aisle)
3. horticultural charcoal
4. peat-based potting soil
6. moss, rocks, and whatever else you would like to add
Add, ta da...
1. 2-3 inches of pea gravel
2. About 1/2 inch of charcoal
3. About 2-6 inches of peat-based potting mix
4. Plant/moss (I totally cheated and just took the moss from outside, LOL)
5. FYI-since mine is an open terrarium, it will need to be watered more often
I know, easy peasy, right?