Choose a mixture of tall, upright plants and smaller, creeping plants and decide how you want to arrange them before starting the planting. For this you should consider the position of the terrarium and the point of view from where it is regarded. It also depend a lot of the shape of the terrarium. So you might want to consider to plant the arrangement with taller plants at the back if the terrarium will be placed in front of a wall and will be seen from the front, or to plant the taller plants in the center if the terrarium will be viewed from all sides.
Being a self-contained, a terrarium must have a 2.5-5 cm layer of drainage material such as clay balls, gravel or pebbles and some horticultural charcoal which helps to maintain the compost fresh. Add a layer of 2.5 cm of moist potting compost. The best choice for this is a lightweight, free-draining but moisture-retentive potting compost. To maintain the soil well aerated you can also add some extra peat substitute or peat.
Use young plants with root systems that are small enough to establish easily in shallow compost. Soak them thoroughly before planting and remove any dead foliage. Remove the plants from their pots and shake off any excess compost. Gently tease out the roots and reduce the size of the root ball to help the plants establish. Insert the plants into the compost, making sure you keep a distance between them so they have space to spread.
If the terrarium is too small to fit your hand in easily then you should use a widger or other small implement to make hollows for the plants and to insert them carefully. Fill in around the plants with more moist compost and firm the surface. Cover bare areas with moss or pebbles to stop the compost from drying out and water lightly by spraying the plants and the moss with a fine mist of water before replacing the lid. Now your terrarium is ready to be displayed.
One established, terrariums require very little or no watering. If excess condensation appears on the glass, ventilate in a little until there is only a slight misting of the glass in the morning.