Check out "The Basics of Terrarium Building" from the Terrarium Man website.
1. Select a container: Anything will work! Look for something dish-like to create an open air container or something bottle-like to create a closed environment terrarium.
2. Place a level of course sand or pebbles in the bottom of the container.
3. If you are making a closed container terrarium, place a level of activated charcoal over the pebbles/sand layer to help filter the air and keep it fresh. (No need if you're using an open top terrarium.)
4. Add a layer of Spaghnum Moss on top to prevent the soil in your terrarium from settling down into the pebbles.
5. Add potting soil.
6. Add the plants and or seeds, and arrange things in a way that is pleasing to the eye.
7. Add other ornaments or decorations!
Read the full instructions at Terrarium Man.
(Images: Terrarium Man)