Restraint and neglect is the best you can do for a Cactus in a terrarium. Why you ask…You have to restrain yourself from caring too much and overwatering your cactus. And they will do wonderful if you just neglect and ignore them for a month or so.
Place them near a very sunny window, typically south or west facing window to make sure they get a good amount of light.
Since the water will not be draining out of the terrarium vessel…water very cautiously. Add about a tablespoon or two of water near the base of the plant every month From spring to fall or every 2 to 3 months in the winter. It is always better to underwater, as you can always add more if needed. If they sit in standing water for a long time time, it will cause roots rot.