Set your container in its final location. Add a 1- to 3-inch layer of pea gravel or pebbles, for drainage.
Add a layer of charcoal, similar to what is used in aquarium filters. This layer will absorb any fumes from decomposing plants from the air.
Place a layer of spagnum moss over the charcoal. This layer will keep the potting soil from dropping into the pea gravel.
Add a layer of potting mix or terrarium mix. Make the landscape interesting by varying the levels of soil to create hills and valleys. Embed a shallow dish or saucer to create a lake in the middle of the forest if you want. Keep in mind how you will view the terrarium once it's finished, and sculpt your landscape accordingly.
Choose plants of varying heights, shapes and textures. Use bonsai trees for the forest and add in miniature varieties of ferns, ground covers, mosses and accent plants. If you are adding stones or driftwood, rinse them first in HOT water to remove all traces of bug infestation.
Set the plants inside the terrarium while they are still in their pots and play with the arrangement. If the terrarium is visible from all sides, place the largest plants in the center. If visible from the front, place the largest plants toward the rear of the container, but not directly against the sides. When satisfied with the appearance, use a kitchen spoon as your shovel and a fork as your rake. Don't loosen the root ball when you plant, since you want the plants to grow slowly.
Tamp down the soil around the plants gently, and add two to three tablespoons of water to the soil. Check the next day for signs of condensation. You want some to appear, but not large droplets of water. If you have watered too much, open the lid slightly to allow for some evaporation. Wipe excess water away with paper towels. If there is no condensation, water slightly and check the next day.