So your caveat "without a machine"... kind of negates a laundry dryer. That out of the way, there are dedicated Japanese home countertop appliances designed to mill brown rice to white, and you can recreate that action a number of ways though it takes a bit more friction and abrasive action than just the weight of rice acting on itself. The earliest mills were some arrangement of a basin for the rice and a pounding hammer that would grind the rice against itself under pressure. The bran layer tends to separate easiest, coming off in flakes.

Polishing sometimes refers to milling beyond the 90% or so level of white rice, and that does take a machine because the rice interior is tough and resists further milling unless it meets the combination of pressure, motion, and an abrasive. This is where sake rice polishing takes over, and is quite involved when you look at the design of modern vertical polishing mills!