Transfer the urad dal and rice to separate bowls and cover with water and set aside for a minimum of 5 hours. If using idli rava, you can soak the rava a half hour before the next step.
Drain the water from the dal and reserve the soaking liquid.
Do the same to the soaked rice/idli rava. Reserve extra soaking liquid rice.
Transfer the urad dal to a high powered blender and make a smooth, fluffy paste using water as needed. It is important to only add a little water at a time to get a nice fluffy texture. Use the reserved water from soaking the dal first. Remember that you can always dilute the batter later if it is too thick, but cannot fix a batter that is too watery easily.
Empty the contents of the blender to another large bowl or other container such as a dutch oven.
Repeat the process with the rice and make a smooth paste. The rice will blend to a smooth paste but will not turn fluffy. Transfer the rice paste to the container with the blended urad dal.
If using idli rava, you can run the idli rava for a minute in the blender. It will not turn into a smooth paste like the rice. That is expected.
Add salt to the batter and mix well. Dilute the batter with water until you have a thick batter. Cover and ferment the batter for up to 24 hours in a warm place. The batter will rise as it ferments, so make sure your container has at least a couple inches of room on top for the batter to ferment.
Check the salt by tasting a drop of the batter. You should be able to tell that there is salt in the batter.
Once the batter has fermented and has increased in volume, stir it back down and transfer to an airtight container and refrigerate until ready to make the idlis.
The batter can be stored in the fridge for up to a week, but it is best used within 2-3 days.