While I have frogs this could be used in any terrarium. I use them to control the humidity in my tanks. Because the ideal humidity levels make happy frogs!
Step 1: It Only Takes a Few Pieces
What you need...
An air pump
Air line tubing
And a bottle I use glass ones that I picked up at a dollar store a while back.
Step 2: Very Simple Construction
Take a length of air line tubing and place it on the output from the Air pump. Next take other end and put the air stone on. Once your done with that stuff the air stone with tubing into the bottle. Fill the bottle with distilled water, plug in the pump and place the bottle where you want it and you are done.