I started with a medium sized cookie jar that I found at Target, but any decorative glass container with a lid will work. I removed the rubber gasket so my little plants could get a bit of air. We used pint-sized wide mouth jars for the kids' projects.
Begin by layering gravel into the bottom of the container, we chose "pretty" gravel from the craft store. Layer the largest stones first and graduate to the finest gravel, this will keep the soil on top and not let it filter down into the stones.
Now, take a walk in the lovely forest nearest you and gather moss and small plants to use inside your jar. I chose a mixture of mosses, tiny ferns, and clusters of clovers.
Fill the jar 2/3's full of potting soil (don't use soil from the walk, it might have little buggies in it!) and arrange your plants. Take care to bury the roots into the soil.
Water with a spray bottle and replace lid. Keep your little terarrium in indirect light and spray with water once condensation disappears from the inside of the jar.