Place the terrarium away from direct sunlight. A terrarium located in the sun will become far too hot for the African violets to thrive.
Lay down a substrate of quality potting soil specifically designed for African violets. Add perlite and vermiculite to the potting soil if desired. Make the layer deep enough to cover the roots of the African violets.
Plant the African violets directly into the substrate in the terrarium. African violets don't have to be left in their pots, although they can be. If you plan to leave the violets in their pots, there will be no need for a layer of substrate.
Place a shallow dish filled with pebbles into the terrarium with the violets. If the violets are still in their pots, position the pots directly on the pebble-filled dish. Otherwise set the dish to the side of the violets. This helps keep moisture in the air and raise the humidity level. African violets love high humidity, and the humid conditions in a terrarium are perfect.
Fill the shallow tray with fresh water each day to replace the water that evaporates in the terrarium. Keep the tray full of water, as this is one of the best ways to keep the humidity level high enough for the violets to thrive.