If you live in a hot place, just cover the batter and leave it in a corner. You can also cover it with a towel. The batter ferments well with in 8 hours.
If you live in a cold place or have central Air Conditioning, you can ferment the batter in the oven or in the instant pot.
Fermenting batter in the oven - this can be done in two ways. You can preheat the oven to 250 F. Turn off the oven, turn the light on and place the batter to ferment over night. The second method is to just turn the light on and ferment the batter over night or atleast 10 to 12 hours. If you are making a large quantity, its a good idea to put a tray under the vessel to avoid over flow.
Fermenting batter in the instant pot - After grinding the batter, transfer it to the instant pot vessel. Turn on the yogurt mode and adjust the time to 12 hours. Close the pot with a steel or a glass lid.
Do not use the instant pot lid to close, if the batter over flows the lid can get stuck.
The well fermented batter will increase in volume and will be frothy and bubbly.