Making a terrarium is fundamentally different from planting seedlings in a gardening pot. The primary difference is that the terrarium is a closed or semiclosed environment, typically made from glass or plastic. Teachers use terrariums to teach youngsters about the water cycle. Most small houseplants make suitable terrarium plants, especially those that thrive in a humid environment. The number of plants in a terrarium varies depending on the size of the container. When selecting plants for your new terrarium, don't overcrowd the terrarium---give the plants room to grow.
Select a container made from transparent material. Suitable containers include fishbowls, fish tanks and large jars.
Wash the container with warm water and soap and dry it thoroughly. An unclean container can introduce harmful bacteria to the terrarium environment.
Place an even layer of gravel between ½ and 1 inch deep on the bottom of the container. This will help with drainage.
Sprinkle 1 or 2 tablespoons of activated charcoal over the gravel. The charcoal helps keep the air fresh and is especially important when you're making an enclosed terrarium.
Add potting soil to the container, and cover the layer of gravel and charcoal. Approximately 25 percent of the container's depth should be potting soil. Use a peatlite mix.
Inspect each plant before removing it from its container, and gently shake off excess soil without harming the roots. Don't introduce a sick or insect-ridden plant to the terrarium.
Make an indentation in the soil and set in a plant, covering the roots and making the soil line the same as when the plant was in its original container. Repeat this process for all the plants.
Spray the plants and inside of the container with water from a spray bottle to remove any dirt, and spray the soil enough to moisten it without actually soaking it.
Decorate the terrarium landscape by adding small ceramic figures.