Wash and soak all lentils in enough water for at least 6 hrs. Since hard lentils are also used, soaking for long help.
After 6 hrs, drain the water and grind in a grinder as you do with your regular dosa batter. The stones might fumble for a while. But pat them gentle, by adding water slowly. Let it grind to a smooth paste, by adding just enough water to get the consistency of the dosa batter.
Remove to a big bowl and allow it to ferment for 2 hrs. Then refrigerate till use.
I had the batter done by 7.30 in the night. It was kept at room temperature till 10.30 or so. It went into the fridge and took it out by 7 in the morning. It was all fine when I made it by 9.30 am.
Heat a dosa Tawa. If you are using a nonstick Tawa, there is no need to spray or apply oil. Check for thinness. The crapes come out well only if it's thin and can be easily spread.
So, add little more water if required. Once the Tawa is hot, simmer to medium, pour one ladle full of batter and spread in a circular shape. Sprinkle oil and allow it to get cooked on all sides. Let the flame be in medium else your dosas will not be crispy.