Wash and soak Rice and Urad Dal together. Soak Soya bean separately. Since dried Soya bean was used, soaked all for about 6 hrs.
Then, grind everything to get a smooth batter. Let it ferment overnight.
When you want to make the dosas, heat a Tawa and pour a ladleful of batter and spread as a circle. Sprinkle oil and cook on both sides.