Wash and soak the millets and urad dal separately for about 1-2 hours
In a mixie, first grind the urad dal to a smooth paste using enough water
Set aside in a large bowl that can hold the entire batter
Then, grind the varagu arisi to a smooth paste with water
Add to the same bowl as the urad dal
Add salt and mix thoroughly to combine
Leave covered in a warm place (see notes if you live in a cooler place) for about 8-10 hours for the batter to ferment
After about 6 hours, my batter had fermented with nice bubbles showing on the surface of the batter. I left it out for another hour or so and then refrigerated the batter