To make a DIY terrarium, you start with an inch or two of rocks in the bottom of the container as drainage material so your plants roots are sitting in water.
Next on top of the rocks add a thin layer of charcoal. This is the best way to absorb any excess moisture, and help keep the little eco-system from becoming moldy. Mold = trouble. We left our charcoal in the bag, or you can open it and add it that way.
On top of the layer of rocks and the charcoal bag, add in some potting soil. Not too much to start, since you want room for the plants and you can add more around them after.
Then add any small accessories, fairy garden accessories, or decorative rocks.
Lastly, give your terrarium a small amount of water and place the lid on. If condensation forms on the glass you can remove the lid for awhile to regulate it.