For making soft idly, add cooked rice along with with rice and urud dal while grinding.
Soak 2 cup rice & 1 cup urud dal in water for 4-5 hours. Grind this soaked white rice, urud dal with 3/4 cup cooked rice with little water. Thus the batter for idly is ready. Keep in mind that not add much water.
Keep the batter in a container for 6-7 hours or until its volume increases before making idly.
For making idly, add enough salt to the batter for taste and mix well