It is best to use a container with a neck wide enough so that you can reach inside with your hands. Wash and dry the container. Use a cardboard tube to gently pour in the materials, to keep the inside of the glass clean.
Since the containers used for terrariums have no drainage holes, a bottom layer of drainage material is needed to prevent water from standing in the soil. You can use fine gravel or coarse sand, in a layer 1.5 to 4 cm deep, depending on the size of your container.
Since a terrarium is a closed, damp environment, you will need to add charcoal to absorb any odors. Cover the gravel with about 1 cm of charcoal chips.
Cover the drainage material with a soil separator, e.g. a piece of synthetic fabric (nylon) or a screen trimmed so that the edges do not touch the glass walls. It will let the water drain through while preventing the potting mix from sifting down into the gravel.
Then add enough potting mix in which to root the plants. Use an appropriate mix: for example, desert plants prefer sandy soil, whereas for tropical plants it is best to add some perlite, vermiculite or peat moss.
There are certain rules of thumb to keep in mind. The base materials (drainage and potting mix) should ideally fill one-third to half the container, depending on its shape and size. The remaining space is for the plants.
Arrange the plants according to their size and growth habits. If the terrarium will be viewed from only one side, place the tallest plants in back. If it is to be seen from all sides, place the tallest plants in the middle.
Be careful not to overfill your terrarium – the plants will need room to grow. Make sure you leave enough space between them. Then you can add some surface decoration: moss, bark chips, stones or sand. Finally, water gently until the soil is evenly damp, but not soaked.