Take parboiled rice and raw rice in a mixing bowl and rinse in water twice or thrice and then soak it in water. Take urid dal and fenugreek seeds in another vessel and rinse it in water twice and add water to soak them.
Leave the soaking rice and dal aside for atleast 4 hours. Add generous water to the dals and rice as the soaking process makes them double their size.
After 4 hours, drain the water from uriddal and add it to the grinder. Initially add 1/2 cup of water to the grinder and start the grinding process. Then slowly sprinkle water in regular intervals and grind the urid dal to a nice and thick batter. This process usually takes atleast 20 minutes. If the urdal quality is good, then we would get more batter. After 20 minutes, our urid dal batter is nice and smooth and if we take some batter and roll it in our hand it should roll like a soft ball. Also the moment we stop the grinding process, we could see small bubbles on top of the batter. This is the right stage to remove the urid dal batter from grinder. Remove the batter and keep it in a mixing bowl.
Next, drain the water from the rice and add it to the grinder. Add water little by little and grind the rice to a nice, smooth and thick batter. Add the rice batter to the mixing bowl which has already the urid dal batter
Mix both the batters well with a ladle or clean hand. When the urid dal batter and rice batter are nicely combined, then close the bowl with a air tight lid or a proper lid to cover the bowl completely and keep it aside for 12 hours or overnight for fermentation.
After 12 hours, we could see batter has fermented and the quantity would have doubled up. Now add salt and mix the batter well. Our Idli / Dosa Batter is now ready for making Idlis / Dosas