Your container will not have drainage holes in the bottom so you’ll want to create a place for extra water to go that keeps it away from your plants’ roots. I often put a layer of sheet moss in the bottom of my terrarium to soak up extra water. I also like the way it looks.
If you don't have moss, you can also start a layer of stones. Put at least 2” layer of stones in the bottom (you do have some flexibility with this if your container is shallow).
Next, using a large a spoon, add an ¼ to ½ inch layer of activated charcoal on top of the stones. The purpose of this is to help drainage and control any odor that might occur.