Soak the urad dal separately in water for 3 hours. Soak the rice and fenugreek seeds together in water for 3 hours (I grind the batter using a Blendtec, if you grind with a grinder, soak the rice for at least 5 hours).

After 3 hours, drain and discard the water from soaked rice and fenugreek seeds and grind to a smooth batter ( little coarse texture is fine) by adding water little by little.

Drain and discard the water from soaked urad dal and grind to a smooth batter and fluffy batter by adding fresh water little by little.

Combine the rice batter, ragi flour, salt and urad dal batter together with a hand. Cover the batter using a lid and allow to ferment for overnight. 

After fermentation, the batter becomes light, and the quantity of batter has risen. Mix well with a spoon once and add water if necessary to adjust the consistency (the batter should not be too thick or thin).

Grease the idli molds with oil and pour a spoonful of batter and steam in an idli cooker for 10 - 12 minutes. Allow the idlis in the cooker for 5 minutes and remove the idlis from molds using a spoon and serve.