1. Depending on the weather I usually soak for Idli Batter. If the weather is warm I soak rice & urad dal overnight, grind batter first thing in the morning and leave it to ferment until night (or until it is fermented enough)
2. If the weather is cold I generally soak rice in the afternoon, grind batter in the evening and let it ferment from evening till next day (approx 12-14 hours)
3. I always leave the batter near a room heater and the batter ferments beautifully
4. When there was Gas Oven in my London Apartment I used to preheat the oven for sometime and then leave the batter inside overnight. If yours is electric oven then you could leave the lights on, let the batter ferment. If you have an instant pot you can use the ‘Yogurt’ setting and ferment the batter
5.I dont like to add Baking Soda to my Idli batter.Have never added it.However if your batter is not fermenting even after 12-14 hours, you could try adding a pinch of baking soda and see if it helps