Steps for Preparation

Soak both rice together and dal separately in bowl to cover them about 2" deep. Soak 8 hours or overnight.
The next morning, drain all the water from the rice and urad dal. Wash two three times and drain and put some in a food processor/ grinder and grind - adding very little water if necessary - to a smooth yet slightly grainy paste. You need to grind both rice and dal separately.
When all the rice-daal mix is ground like this, put it into a large mixing bowl and add enough water to make a batter.
Keep the batter aside in a warm, dark spot, covered, for 6-8 hours.
After this fermentation, stir the batter well and add salt.
when your are ready to make idli grease idlimold with little gingerly oil.
Add little less 1/4 cup batter in each mold
Cover and steam it for about 12 minutes or until tooth pick comes out clean.
Carefully remove each using small spoon.
continue to make idli until required numbers.
Serve with chutney and idlimilagaipodi or sambhar