You will need a safe substrate. Safe meaning: One, it contains nothing small enough to be accidentally ingested resulting in impaction. You should therefore not use gravel, small rocks, sand, wood chips or shavings, etc. Two, it contains nothing that can be toxic to the frog. You should therefore not use soils containing fertilizers or pesticides, printed newspaper, printed paper towels, etc. You can use plain white paper towels, non-printed newspaper (available at furniture moving and truck rental companies sold as packing paper), reptile carpet, fertilizer/pesticide free topsoil, composted leaves, or ground coconut husk (sold at pet stores as Bed-a-beast, Forrest Bed, Jungle Mix, etc).