Place the oats in a blender or food processor, and process until the oats are finely ground. This takes about 30 secondsin a blender, but could take up to 2 minutes in a food processor. Stop and scrape down the sides, if needed, to make sure everything is processed evenly into a fine flour.
If you use a food processor, I recommend pouring the flour into a fine mesh strainer and sifting the flour into a large bowl, to make sure you strain out any large pieces that might not have been ground well. This way you'll have consistent measurement with your flour in the future. (This is optional, though. Small pieces of oats in your recipes should also be fine!)
Store the oat flour in a dry, airtight container for up to 3 months. You can extend the shelf life slightly longer by storing this in the fridge, but if you don't make too much at one time, you should have no problem using your flour that fast!
2 cups of rolled oats should make roughly 1 3/4 cups of oat flour. The nutrition information is for roughly 1/4 cup of flour, but this is just an estimate, and not a guarantee. If you use steel cut oats, you may get closer to 1 1/2 cups of flour for every 1 cup of oats, since they are denser than rolled oats. But, in that case I recommend using a high-speed blender for grinding the oats, since they are tougher than rolled oats on a food processor.