In a food processor or nutri bullet with the grinder attachment. Add the oats and blend to a flour.
Store in an airtight glass jar with a seal.
You can use any kind of oats, but I like the steel cut one. I like to store mine in a glass jar with a rubber seal, but I have also bought the bag of oats that are in a resealable bag and I make the flour and put it back in the bag. QUICK SHOPSteel Cut Oats