Its enuff if the dal and rice soak for 2-3 hours. Even an hour will do. Grind them to a fiiiiiiiiiiiiiiiiiiiiiiine paste. DO NOT add too much water just to make it fine. Keep the batter thick and yet finely ground. Meaning you'll have to run the grinder for a loooooooooooooooooong time. Ditto with rice too. So for about 4.5 cups of urad dal you'll use about 0.5 cups of rice. Grind the rice and urad dal separately and then mix them together. Now add the following to the batter. The coconut is optional, and if you don't have soda, don't despair. Just leave the batter COVERED for about half hour so that it ferments slightly. Now comes frying. Take batter into hand or use tablespoon if you are comfortable with that and deep fry spoonfuls of batter in hot oil. Fry till the vadas are a light brown and remove. Immediately after you remove them from the oil, dunk them into some water at room temperature. Keep a bowlful of water handy beforehand. They'll make a chooooooiiiiiiiii noise. Make sure they go completely underwater for about 2 minutes. If you touch them they should lose crispiness but not crumble. Do this to all the batter. Arrange all the vadas one next to other in one layer. All vadas won't fit, so put only bottom layer. Now dunk dahi in a bowl, and using a fork beat it thoroughly to remove lumps. Add some chopped green chili, salt to taste, red chili powder to taste, chat masala for flavor. Now dunk this beaten dahi over the first layer of vadas, and then place next layer of vadas. Add dahi over them again, and continue till all the vadas are over. At the very top put lots of dahi. Decorate with red chili powder and chaat masala and finally sprinkle with chopped coriander leaves. Now this itself can be served, but if you want it to be perfecto, then make some coriander chutney by grinding green chillies, coriander leaves and lime juice and salt together. Also make some sweet chutney using tamarind and sugar and red chili powder and salt. Before serving dunk lil bit of both chutneys over the vadas and dahi and ... finito!