American Profile offers a concise guide on getting started with terrarium crafting. As always- I highly reccomend the charcoal layer:
Charcoal filter. A layer of activated charcoal as used in aquariums will filter the air and water and keep them sweet. You can buy charcoal in pet shops or nurseries. If your terrarium is an open container, you won’t need the charcoal filter, but you will need to water your plants, since the system won’t be enclosed and self-watering. A layer of sphagnum moss will keep soil from sinking into the lower layers.
How to Build a Terrarium at American Profile