Clean rice and urad dal and wash them thoroughly . Soak in water for 4-5 hours or overnight, separately. Also soak fenugreek seeds in dal.
Strain excess water from urad dal and grind finely using very little amount of water. Strain excess water from rice as well and grind coarsely using less water.
Mix both the paste in a big bowl and prepare a thick dough (it should not have pouring consistency). The consistency should be same as required for making pakoras.
Cover and keep aside on warm place for 12-14 hours to ferment. It takes less time to ferment the batter during summers but during winters it takes around 20-24 hours.
Once batter is fermented it gets puffy. Batter for making idli is now ready. Store it in refrigerator and use within 4-5 days.