The beauty of grinding oatmeal -- or steel-cut or rolled oats or even oat groats for that matter -- is that you can create tiny portions of oat "flour" that stay fresh for your immediate needs. You can then mix your "just in time" inventory of ground oatmeal in smoothies or gluten-free baked goods. Grinding your own also means fewer specialty items to keep track of in the pantry, as you won't need a dated and aging bag of oat flour on the shelf. Oat grains are much softer and more fragile than wheat grains, so a coffee grinder or similar kitchen gizmo works fine for this task.
Measure a quarter to a half cup of oatmeal and place it in an electric spice or coffee grinder. You can also use a food processor, a blender, a blender grinding attachment or a grain mill. Quick-cooking and old-fashioned oatmeal work equally well.
Pulse the grinder or processor until you have the texture you are looking for in your recipe. Thirty seconds results in a fine powder.
Sift the resulting flour through a fine-mesh strainer and return any large pieces to your grinder along with your next batch of oats. A few coarser grains will lend a pleasantly variable texture to your baked goods, but they're unwelcome in a smoothie.