Put 1 3/4 cups of oats in your blender and turn it on until the oats grind down to the texture of flour. (This will take about 1 minute depending on the power of your blender).
Pour the oat flour into a wire mesh strainer or a flour sifter and sift over a large bowl to take out the large pieces of bran. (Discard the large pieces of bran or use in another recipe).
Measure out 1 1/2 cup of oat flour and put it into a bowl. (This should be what you have left after sifting out about 1/4 cup of bran).
Add the tapioca flour and xanthan gum to the oat flour and stir well until it is well mixed.
Store in an airtight container in the fridge until you are ready to use it in any baked goods.
Notes
You want to make sure to measure out exactly 1 1/2 cups of oat flour for the mix.
Starting with 1 3/4 cups of whole oats, it should yield 1 1/2 cups of oat flour once it is ground and sifted.
Xanthan gum is optional, but it will help your baked goods hold together a little better and not crumble.
Make sure to tap the measuring cup a few times to settle the flour when measuring or measure the flour by weight.