Pour about an inch of pea-sized gravel into your jar, followed by 1/2 inch of activated charcoal. Add about 3 inches of soil on top of the charcoal. This layering helps keep your terrarium drained and clean.
Choose your terrarium plants. Rosemary, basil, thyme and chamomile work well for an herb terrarium. Small succulents like colorful cacti and little yucca plants work well. You may also opt for just a little moss and some shamrocks, bluebells or Irish moss.
Gently open up small holes in your potting soil for your plants. Slip the roots of each plant into a hole and cover the roots with dirt. Space the plants at least an inch apart. If planting shamrocks, push the bulbs about 1/2 inch into the soil.
Water your terrarium with about a cup of water. This water should continuously evaporate and fall on your terrarium, meaning you'll only need to water it about once a month.
Drill four or five small holes in the top of your jar lid and screw it into place. This allows oxygen into the jar and prevents mildew while keeping the terrarium humid inside. Place the jar in a windowsill that gets bright, indirect light for most of the day; this should meet the light requirements of most terrarium-sized plants.