Building the layers
Begin by creating a drainage area of pebbles and charcoal by covering the bottom of the container with one to two centimetres of sand or pea gravel; this helps keep excess water at the bottom of the terrarium. Next, carefully add a seven-millimetre-deep layer of crushed charcoal to absorb unpleasant odours.
To keep soil out of the drainage area, cover the pebbles and charcoal with a piece of synthetic fabric. Recycled nylon fabric, say from stockings or a curtain remnant, is ideal because nylon cloth will contain the soil but allow water to pass through, plus it'll decompose slowly. Place eight to ten centimetres of sterile, slightly damp potting soil over the nylon. Using a spoon, level the soil or mold it into contours and valleys.
Planting the terrarium
Place the largest specimen near the centre if you plan to view the terrarium from all sides, but plant the tallest in the background if you plan to view the terrarium from only two or three sides. Making sure no leaves will touch the sides of the container, which may cause them to turn brown, place each plant in a small hole and gently firm the soil around its roots. Just for fun, you can also add a small figurine or two.
If the glass is dirty, spray it lightly with water and wipe it with a piece of cotton wrapped around the handle of a wooden spoon. Cover the terrarium and place in a bright area, but not in direct sunlight, such as in a north- or northeast-facing window. Direct sunlight can burn plant leaves
Caring for the terrarium
Check the terrarium daily for a week. If water condenses heavily on the inside glass, remove the lid to let excess moisture evaporate. If the plants wilt, and no condensation forms, add a little water. After that, water only when the soil feels dry. Don't fertilize unless the plants start to turn yellow, and then only at one-fourth the recommended rate. When the plants are as tall as you want them, pinch off any new growth. This will also encourage them to grow bushier. Remove any faded flowers to prevent disease. Once a year, add nutrients and freshen the terrarium's appearance by scraping off the top layer and adding more potting soil.
Related web sites
Black Jungle Terrarium Supply
A Terrarium Resource Guide
Plastic Bottle Terrarium