To make a terrarium, layer small pebbles, activated charcoal and damp potting soil in a glass container. Place small plants in the soil and mist them lightly with water.
A glass container with a wide opening works best for a terrarium, as the wide opening makes it easier to plant. You can use a container with or without a lid. Closed terrariums, or containers with lids, retain moisture better and require little if any watering. Before adding layers, clean the container with hot, soapy water to remove contaminants.
Place a 2-inch layer of small rocks in the bottom of the container. This rock drainage layer is necessary since the container doesn't have drainage holes like a regular flower pot. Add a 1/2-inch layer of activated charcoal to help with drainage, control odors and remove potentially toxic chemicals for plant health. Pour in about 1 1/2 inches of damp potting soil.
For your terrarium, choose a plant that thrives in humid conditions. Examples of plants that grow well in terrariums include pothos, baby's tears, creeping fig, button fern and croton. Make a small hole in the soil for your plants. If your hand doesn't fit inside the container to dig a hole, use a wooden skewer or long-handled utensil. Lower the plants into the holes, using two skewers if necessary.