Soak the black gram overnight or soaking for 5 -6 hours is also enough. I generally soak overnight.
Wash it well under running water and remove the skin while washing. Keep it aside.
In a mixer or a wet grinder, add the soaked lentils and grind to a fine paste while adding water little by little. Once it becomes frothy and fine, transfer the contents to a large bowl.
Wash the idly rava several times before using and squeeze to remove water from the rava.
Now add this rava to the fine batter of black lentil and mix well. Keep it aside for 4 – 5 hours for fermenting.
Once fermented and ready to use, add salt and mix well. Add salt only to the batter that is being used to make idlis at that point of time. The unused batter can be kept in the refrigerator.
Grease the idli moulds with ghee or oil. Transfer little amounts of batter to mould and steam cook for 10 mins.