To make a terrarium, layer rocks, charcoal, potting soil and plants in a clear glass container, place in indirect light, and water intermittently. Optionally, add decorative elements such as additional rocks, pebbles, shells or pinecones.
To make a simple terrarium, decide which plants to include based on how they thrive in different environments. Choose all succulents or cacti if using an open container, and all moss or ferns for a closed container. Pair plants that thrive in the same environments for best results, and try to get small plants that will not grow too big for the container.
Start with a layer of marble-sized rocks in a clear glass container such as a goldfish bowl, cookie jar or brandy snifter. Add 1/2 inch layer of activated charcoal to filter the water and prevent fungi growth. Fill the container halfway up with sterilized potting soil, and plant your plants in the soil so they do not get uprooted easily. Optionally, add decor such as ceramic animals, garden gnomes or additional rocks or pebbles. Add moss around the decor if desired.
Add a lid if needed, and then set the terrarium in indirect sunlight. Water intermittently, but make sure not to water-log the roots. If water condenses on the lid, remove it to air out the terrarium, then replace it.