Firstly, wash, soak rice in a vessel for 5 hours. Urad dal in another vessel for minimum 3 hrs.

Drain water, grind urad dal first. Use ice cold water for grinding.

After this, sprinkle water at regular interval as needed.(Will need water more or less depending upon the quality of dal). The batter will be light, fluffy and have microbubbles when seen keenly.
Tip: You can keep a bowl of water and drop a teaspoon of batter in it. If it floats, you have ground it enough.

Collect the ground urad dal batter in a big container, that is enough to hold the raising fermented batter the next day.

Add rice the same grinder Add more water when the grinder runs slowly and the batter gets heated. Grind till it is smooth or a little coarse (whichever consistency you prefer, I personally grind smooth). Add more water if its needed.
It takes about 25-30 mins approximately. Time may vary, according to make and model of grinder.

Transfer this rice batter to the urad dal batter and add salt and mix well with your hands.

I use cloth in it to cook the idli, you can make with the ordinary idli cooker too. When you use idli plate without cloth in the idli cooker, grease the idli plates with sesame oil before pouring the batter. After it gets cooked, you have to take it out and let it sit for 1 minute so that the idli comes out without sticking to the plate. Steaming idli will take around 8-10 minutes.

Now ready to serve with chutney, sambar..