Choose a clear plastic or glass container, such as a fish bowl, glass jar or jug, or a container made especially for the purpose. Avoid colored or tinted glass, which will block sunlight. If you plan to grow plants that require high humidity, choose a terrarium that can be covered. Keep in mind that closed terrariums will be more susceptible to disease. Wash the container with warm, soapy water, and rinse it several times to be sure any trace of detergent has been removed.
Put damp commercial potting mixture in the bottom of the container. A general rule of thumb is to measure the width of the container, and use approximately 1/4 inch of potting mixture for every inch of width, up to 3 inches deep.
Choose plants for the terrarium. Slow-growing, dwarf plants are preferable, but larger plants can often be cut back to keep them small. As a general rule, terrarium plants shouldn't grow more than 12 inches tall at maturity. Avoid mixing plants with different needs. Tropical, humidity-loving plants shouldn't be combined with plants that prefer a drier environment. Choose plants that will be interesting because of differences in color, size and texture, but keep in mind that terrariums are too moist for most cacti and succulents.
Remove the plants from their containers. Start with the taller plants, and plant them in the back of the terrarium. Plant shorter, or rounded plants nearest the front. Don't crowd the plants or plant them to near the walls of the terrarium. Try to avoid getting potting mixture on the leaves, but any mixture that does end up on the leaves can be removed with a spray mister or an eye dropper filled with water. Don't use more water than possible, as the leaves won't dry as quickly in a terrarium.
Place the terrarium near a sunny window, but avoid placing it in direct sunlight. Heat will build up quickly in the terrarium atmosphere, and can overwhelm the plants. Temperatures in the room should be around 75 degrees Fahrenheit during the day, with nighttime temperatures about 10 degrees cooler. If you have filled your terrarium with woodland-type plants, the temperature should be around 10 degrees cooler overall, with daytime temperatures around 65 degrees Fahrenheit.
Water the terrarium sparingly. Keep an eye on the terrarium the first few weeks to determine how much moisture it will need. If your terrarium is covered, it may not need to be watered for three or four months. After watering, open the cover slightly if more than a light film of condensation forms on the inside of the glass. Open terrariums will need more water than covered terrariums, but much less than plants grown normally.
Begin feeding terrarium plants a weak fertilizer solution one year after planting. Use a liquid fertilizer for indoor plants as directed on the label, but dilute the solution to a tenth of the label suggested strength.
Decorate your terrarium with non-plant items, if desired. Stones, wood chunks, bark, or ceramic figurines can all be used, but be sure the items are clean so that bacteria and insects aren't introduced into the terrarium.
Keep the terrarium neat and tidy. Remove dead leaves immediately, and replace any dead plants with fresh plants.