Add a 1- to 2-inch layer of pebbles to the bottom of the container for drainage (image 1).
Add a 1-1/2-inch layer of activated charcoal over the pebbles (image 2).
Add a 1-1/2-inch layer of sphagnum moss over the charcoal. This will help prevent soil from moving down between the pebbles.
Pre-moisten the potting soil until damp but not soggy, and add three to four inches to the bottom of the container. The amount you add will depend on the size of the container and the type and size of plants that you want to grow.
Remove plants from their pots (image 1). Remove excess soil from around their roots, and cut away pot-bound roots. If a plant is too big for its place in the container — either in size or root mass — divide the plant at its roots, if necessary, to fit the size and scale of the container. Create a planting hole in the soil (image 2) and place each plant where the leaves won't touch the glass; firm the soil around the base of each plant.
Lightly spray the foliage and inside of the glass to remove any soil particles.
Finish off your miniature garden with stones and decorative pieces as desired.
Place in a bright spot indoors but out of direct sun, and leave the top on for a day or so until the leaves and glass dry. If the soil feels soggy, remove the top of the terrarium and let the excess moisture evaporate. Then put the lid back on the container.