All you need for this is Nethra pazham / plantain. I have found them in most vegetable market - Indian or otherwise. Cut the plantain into 2 or 3 pieces. Cut the edges off too. In a Pressure cooker add 2 cups of water and place the pieces on the 2nd idli thattu.Pressure cook upto 4 whistles. You can also use other methods of steaming - using the rice cooker with steaming options etc.
Let it cool and then open the cooker. Remove and serve.