Clean the terrarium with soap and water. Rinse thoroughly and dry it completely.
Place gravel mixed with charcoal along the entire bottom of the terrarium. Use about 1/2 an inch, and sand is an acceptable alternative. Spread the gravel or sand out equally and look at it from the side to ensure that the layer is flat.
Dump 1 inch or more of potting soil on top of the gravel layer. Make each layer separate; do not mix them together. Pour water onto the soil just enough to make it moist. Do not saturate the soil.
Plant your ferns or other plants around the terrarium and space them out. Put another 1/2 inch of potting soil in the terrarium and pat down firmly around your plants. Use scissors to trim any leaves that stick out over the top of the terrarium. Add moss, stones, plastic animals or drawings of animals, if you like.
Pour more water into the terrarium, enough to saturate the soil. Spritz the plants with water to moisten the leaves with a light coat of water. Cover the terrarium with a glass top so it traps the heat inside.
Seal the edges with Scotch tape. Select a warm place to put the terrarium, preferably a location with partial sunlight. Water the plants every few weeks.
Monitor the terrarium during the course of several weeks. Take notes on how the plants grow and the effects of water and sunlight. Record average temperatures and take pictures. Modify the watering schedule or temperature and then record how that affects the plants or set up more than one terrarium and use different heat and water conditions to observe what works best for healthy plants.