Spread 1 to 2 inches of coarse gravel or stones in the bottom for drainage.
Add a thin layer of activated charcoal.
Add 2 to 3 inches of light potting soil.
Plan the layout on a plate or a sheet of paper, with tall plants behind short ones and room to grow.
Take each plant out of its pot, shake off extra soil, loosen the roots, set it in, and press the soil firmly around the base, using long tools if needed.
Water lightly, by misting or with spoonfuls.
Finish with sand, gravel, stone, or moss and add decorations.
Put the lid on.
Notes
Gift idea. If moisture collects on the glass, open briefly. Well-sealed terrariums may not need water for months. Keep in bright light but NOT direct sun (glass magnifies heat). For cacti and succulents, leave the lid off and water as needed.Safety: Glass containers and small decorations: adult help for kids; figurines and stones are choking hazards under age 3. Wear gloves and a dust mask with dry potting mix and charcoal dust.