Brown rice flour is nothing more than brown rice grains ground to a powder. The challenge is that rice is difficult to grind into a smooth flour. Even carefully ground rice can be gritty, like meal, rather than smooth, like flour. To grind rice to a smooth flour, you must have the right tools: either a powerful blender or a home grain mill. Coffee grinders and standard blenders can make rice meal, but they will not give you smooth flour.
Measure 1 cup of brown rice into a high-speed blender. Put the lid on the blender.
Turn the blender on to a "Medium-High" setting.
Scrape down the sides with a rubber spatula, if necessary.
Listen to the sound of the rice hitting the sides of the blender. The tinkling sound of bits of rice striking glass or metal will become quieter when the rice is reduced to a powder.
Place the rice in the hopper of your grain mill.
Set the grinder to "Medium" and grind the rice into a coarse meal.
Put the meal back into the hopper.
Set the grind to "Fine" and grind the meal into a flour.