If you are going to ask me why the heck would anyone really have to make rice flour at home when you can just get a pack of it off the shelves in any grocery store, my answer would be, I really do not know. While there is nothing wrong in buying from shop, I, for one have never bought rice flour from shops, yes never. If any of the elderly ladies ever come to know that I indeed have, they would have my scalp. As it is so easy to prepare, I've always preferred homemade rice flour not just for making the South Indian bakshanams (savories/snacks) but also for making a lot of other recipes with it.
To make Rice flour at home, always buy Maavu Pacharisi (Raw Rice). It is a different variety of raw rice from the one that we eat daily for lunch. It is usually used for making ven pongal or sarkarai pongal.
Pick, clean and wash rice, rinsing it twice or thrice. Soak in plenty of water for 2-3 hours. If not possible, you need to soak at least for 1 hour. Drain the water completely. Spread a clean cloth (preferably cotton) on the floor, transfer the soaked rice to the cloth and spread it well on the cloth. Let it air dry completely. Make sure that you dry it in shade. The time for drying depends on the climate and quantity of rice. I always do it inside my home and not in direct sunlight.
After it dries, the texture of the rice would be brittle and crisp. If the quantity of the rice is just 2 cups or less than that, grind it to a powder using the mixie jar, but if it is a kg or more, better grind in the flour mill. Sieve after grinding to remove any leftover grains. Store in an airtight jar and use as required.
Why should you soak the rice for such long hours? It is because the more water the rice absorbs during soaking, the more flour you get, which is much more economical than the store bought flour. Also, the bakshanam (Savory/snack) that you make with the flour, stays crisp and crunchy for a long time when stored.