Soak urad dhal & rice separately for about 6 hours. Drain the excess water. Grind the rice into a coarse paste by adding some water. Then grind the urad dhal separately into a smooth paste. Mix both the ground paste together and add salt to taste.
Cover and keep in a warm place overnight to ferment. Next day, add the baking soda and mix well.
Grease the idli plates (idli moulds) with little oil and pour 2 tablespoons of the idli batter in each mould. Pour a cup of water in a pressure cooker and place the idli moulds in it. Steam cook for 15-20 minutes. To check whether it is cooked or not, touch one of the idlis with your finger if it comes out clean its cooked. Remove them using a spoon. Serve warm with any of the chutneys (coconut, tomato, mint or coriander chutney) or sambar.