Cook your sausage over medium heat, stirring frequently, and covering with lid so to keep as much of the juices in the pan as possible. Stir in flour, and then add milk. Cook at medium  to medium low heat until gravy thickens. Add about a tablespoon of flour at a time to thicken, or conversely add about 1/4 cup of milk or dairy alternative at a time to thin out. The more you heat it, the more it will thicken, so once you achieve your desired thickness either keep on very low heat, or turn off for serving.