Step 1. Choose an open or closed glass container of some sort.
Step 2. Cover the bottom with an inch-thick layer of pebbles or rocks to create a false drainage system for the plant roots.
Step 3. Add a thin layer of activated charcoal keeps the water fresh, and prevent mold and bacteria from building up.
Step 4. Add a layer of potting soil. They also make a special mix if you choose to use cacti and succulents instead of moss and ferns.
Step 5. Place plants in the terrarium, starting with the largest plant first.
For more details, check out this terrarium guide here.