Put simply, you use sodium bentonite or calcium bentonite (the former is sold as many brands of absorbant cat litter) in water, mixed with something else, such as red earth clay, milled sphagnum moss, peat moss, etc. You use a minimum of about 30% of the bentonite to the other ingredients, making sure if you use sodium bentonite that you soak it for a day or so first, to make sure it becomes saturated with water. You make your wet mix and spread it on the glass of the terrarium. Give it an hour or two and it is good enough to stand upright and you can plant straight into it.

This thread has some more insight into the process with a slightly different recipe: http://www.frogforum.net/gallery-dar...mantellas.html