Add your oats to your blender or food processor. Do not fill the container more than halfway; leaving empty space allows the oats to circulate freely so every flake gets pulverized evenly.
Secure the lid and blend starting on low speed and gradually increasing to high speed. If using a blender, run it for up to 60 seconds. If using a food processor, let it run for 1 to 3 minutes, stopping occasionally to scrape down the sides or stir the oats.
Stop the machine and rub a bit of the flour between your fingers. It should feel like a fine, soft powder with a very slight texture. If you notice any large flakes or sand-like grains, blend it for another 10 to 15 seconds.