Choose a glass container large enough to hold the variety of herbs desired. Allow for 3 to 5 inches above and around the plant at its mature size. Any large glass container can be used as a terrarium, provided it is capable of ventilation as needed.
Wash the glass container thoroughly and dry it. Running it through a dishwasher with a sanitizing cycle is ideal, since it will kill any harmful bacteria.
Create a 1/4- to 1/2-inch layer of small pebbles or coarse sand along the bottom of the container. Adding a stratum of coarse material at the bottom of the terrarium provides adequate drainage to maintain a healthy balance of moisture in the soil.
Lay a 1/4-inch layer of sphagnum moss on top of the pebbles or sand. Sphagnum moss creates a permeable layer that allows moisture to move through it while holding the soil in place. It also adds acidity to the soil.
Fill the terrarium to the halfway point with potting soil. Organic potting soil is best if the herbs are intended for human consumption, but any garden soil free from chemical fertilizer will work.
Moisten the soil to settle it and prepare it for planting. Using a spray bottle is best because it maintains a level surface and prevents the soil from becoming waterlogged. The soil is moist enough when a small amount of water begins to pool in the pebbles at the bottom of the container.
Using the tip of your finger, create a ring of small depressions around the center of the soil. Place one or two seeds into each depression, and pinch the soil closed. The depth will be determined by the type of seed used. For example, basil, dill, and summer savory seed should be planted at a depth of 1/4 inch, whereas chamomile seeds must be lightly pressed into the top of the soil to allow them to receive light for germination.
Mist the seeds daily. Because terrarium gardens maintain moisture more effectively than other forms of container gardens, only a small amount of water should be used. If misted too frequently or heavily, the terrarium will develop algae or fungal growth, which will damage the health of the seedlings.
Harvest leaves once the plants have reached 3 to 4 inches in height. Do not harvest too vigorously, or the plants will be damaged and might die.