Soaking with enough water ensures dal and rice both gets double in quantity
of water and grind in high speed for a minute, add 1/4 - 1/2 cup of water again and grind until smooth, urad dal batter will be soft, fluffy and smooth if grind properly with enough water. If your mixer start making noise add more water.
If your weather conditions is cold it might required another
Off flame and allow it to cool, take out the idli stand and sprinkle little water on each plate, use a spoon to scoop out idli