You need to pour enough water into a wide vessel and bring it to a boil. You must place the batter-filled tumblers inside once the water boils and steam starts coming out. You should make sure that water does not enter the tumblers. You must cover the vessel and steam it until the idlis cook fully.
You can set the steaming time based on how long you usually take to make regular idlis. The exact time might change depending on the tumbler size, batter quantity, and steam level. So, you should always check if the idli is fully cooked instead of just relying on the clock.