Firstly, in a large bowl soak 1 cup urad dal for 4 hours,
drain off the water and transfer to blender or grinder.

Blend to smooth and fluffy batter adding water as required,transfer the batter to a large bowl. keep aside.

Now in another bowl take 2 cup idli rice,rinse the idli rice with enough water and drain off the water

Repeat this for 2 or 3 times or till water runs clean.

Squeeze off water from idli rice and add to urad dal batter.

Mix well making sure rice and urad dal are combined well.

Now cover and rest in warm place for 8-10 hours or till the batter ferments and doubles.

After 8 hours, batter doubles indicating well fermented with air pockets present,
add 1 tsp salt to the batter and mix gently without disturbing the air pockets

Scoop the batter into idli plate greased with oil,place in steamer and steam for 10 minutes on medium flame or till a toothpick inserted comes out clean.