Soak dal for around 4-5 hours.
Grind it to a very smooth paste (Do not add too much water, just add as much as needed). Touch the batter and no pieces of dal should be felt. The more it is ground, the better. Once it is ground very soft, some bubbles can be found on the surface, the more the bubbles, the better is the batter.
Wash the rava with water(if rava is prepared at home, no need to wash it again) and grind it for 5-6 seconds, do not grind more than that, just to make the rava a bit fine. Some people, do not grind the rava, but most of the times the ready made rava is too coarse and grinding it makes the idlis turn out softer.
If idli rava is not available, use 2 cups of rice instead. Soak rice for around 4-5 hrs. Then let the water go by spreading it on cloth for around 30mins. When all water is gone, grind(without any water) to a coarse powder (of coarse sand consistency).
If you have more time in hand, grind the dal previous day afternoon and mix the rava in the night. Add salt and mix well (When dal is completely ground, the rava can be added to mixer and ground 5-6 seconds as explained earlier to speed up the process. But make sure the dal is completely ground before adding the rava).
Keep the batter in a warm place overnight. I keep it on warm gas stove, after all the cooking is been finished.
Check in the morning if the batter is fermented. The volume would have increased. If it looks same as the previous day, it is better not to steam it, the idlis become like stone and everything has to be discarded. Instead make some dosas with the batter. There is no point in keeping it for one more night, because the idlis will become sour.
Grease the idli stand with oil. Steam them for around 10mins (if using cooker, do not use the weight). Leave it for around 5-10mins before opening the steamer/cooker (or else, the steam falls on idlis and make them mushy).