Wash the rice and lentils (urad dal and fenugreek seeds) separately, and keep them aside in different vessels.
Soak rice in water. You can use water just enough to cover the rice.
In another vessel, soak the lentils. Again, use water just enough to cover the lentils.
Keep them closed for at least 6 to 8 hours. You can even keep them soaked up to 12 hours.
Soaking the rice and lentils in water for hours, will result in fermentation of the grains and dal.
Once they are soaked, you do not have to remove the water as you can use the same water to grind rice and lentils.
Grind the rice in a blender along with the cooked rice, then grind the dal separately. If you are making the batter for idli, make the batter with very little water (just enough to make athick batter) since its not possible to make idlis with a runny batter. If you are using poha instead of cooked rice, you have to soak the poha for about 30 minutes prior to grinding the ingredients. You can soak the poha in the same container with rice, but soak it just thirty minutes before you grind the contents. Adding cooked rice or poha will make the idli (or dosa) more softer in texture.
Mix both the grind rice and grind urad dal, add salt and stir well to get the idli batter.
The mixture should be thick enough to pour them into the moulds and make idlis. You need to keep this for fermentation for at least 8 to 10 hours or overnight, after which the batter will increase in volume so make sure to use larger containers to keep the batter. Once the batter has increased in volume, you can mix well and use it for making idlis.
This is how you can prepare the best idli batter recipe at home. To get the best results, try to use the idli rice itself and very fine quality ingredients, also make sure you follow the right proportions and directions as mentioned in the recipe here.