Remove your plants from their pots. You may find that they are root bound in which case you will want to tease the roots apart, even cutting some off. By cutting off some of the roots, called root pruning, you will also retard your plant’s growth, which is usually a good thing when growing plants in the confines of a terrarium. You'll also want to take off any excess soil as well.
Some plants can be separated by gently pulling them apart. If you’re not sure, look at the base of the plant; there may be several small plants grouped tightly together even though it just looks like one plant. This can come in handy if the plant looks too big once you've tried it out in your terrarium.