How it works:
Creating a closed terrarium is truly creating your own mini ecosystem to bring home. After you finish your master piece add water to start the water cycle. A heavy mist is fine or pouring a few oz. The water is then absorbed by the roots. During the process of photosyntheses, water will evaporate through the leafs pores which will cause condensation on the glass of the terrarium. Water drops back down into the terrarium.
When to water:
Technically you shouldn't have to water it at all. The plants will continue to cycle the water. However, there are some cases the water runs dry. In that scenario, water just enough to moisten the soil to restart that water cycle.
Tip: If the glass has condensation on it then it has enough water. If you notice yellow/mushy leaves then it may have too much. You can open the container to allow some of the water to leave then close it again. If there is no condensation on the glass then there may not be enough water and will need to add more. It is better to add a little at a time.
Keep away from heating sources and direct sun. Keep in an area with bright indirect light and moderate temperatures.
If there is a sign of mold (white fuzz), remove it immediately. Spraying with alcoholic/water (50/50 mix) will help keep things under control.
If the plants begin to overgrow their containers, you may prune them back or replace them with a smaller plant.