A closed terrarium develops its own eco-system, a mini version of the planet's water cycle. Once established, they should be virtually self-sustaining and very low maintenance.
You need to establish the right balance of moisture in your terrarium for it to be successful. An amount of condensation on the glass during the day, usually in the morning, is normal (and essential) but if it's excessive and never clears, there may be too much water inside your terrarium. If this is the case, take out the cork for a few hours or overnight and let some moisture evaporate away. Replace the cork and monitor the terrarium again until the balance seems right.
On the other hand, if you never see any condensation on the glass, your terrarium might be too dry. In this case, you can give your plants a small amount of water, preferably with a mister. Just a couple of squirts should be enough. Replace the cork and watch the terrarium again for a few days. Only give tiny amounts of water at a time - you can always add more later but over-watering can create problems for your terrarium.
Keep your terrarium out of direct sunlight as you could overheat your plants.
If you need any help or advice with your terrarium, please get in touch.