Put 1 3/4 cups of gluten-free whole oats into a blender and blend for about 1 minute, until the oats turn into flour.
Pour into a wire-mesh strainer or a flour sifter, and sift over a large bowl until all the fine flour has fallen through. (Throw away the large pieces of bran that are left in the strainer.)
Add tapioca flour, xanthan gum, baking powder, and salt to the oat flour and then give it a good stir.
If you wish to add the shortening, add all the dry ingredients along with 2 tbsp of shortening to a food processor and blend until incorporated.
Store in an airtight container for up to 3 months and use it as you would Bisquick.
Notes
I have noticed that recipes occasionally require slightly more liquid when using this gluten-free Bisquick with homemade oat flour vs. store-bought flour.
Homemade oat flour tends to have more bran than store-bought oat flour, and it works better when measured by weight rather than volume.
Make sure to mix it well so the proportions are evenly distributed.
You can leave out the shortening to make it fat-free.