Best way to prepare this is when you grind your idli batter. Even otherwise soak the urad dal as you would for Idli for about 4 hrs. Grind in a grinder / mixer with just enough water.
Mix the wheat flour in 1 cup of water and add to the urad dal. Run it for couple of times for getting a mixed batter. Add salt and mix well.
As I said if you want it fermented, keep it at normal temperature. Else, refrigerate. Fermented batter tends to become soft.