In a large skillet over medium-high heat, crumble and brown the sausage. Don't just cook it, but actually allow it to brown some. It adds flavor. Do not drain the grease away.
Sprinkle the flour over the sausage and stir. Cook for 2 to 3 minutes, stirring constantly.
Gradually add the milk, stirring well after each addition. Reduce to a simmer and allow the gravy to thicken, stirring frequently. For thicker gravy, add about 2 cups total. For thinner gravy, add more milk. Add salt and lots of black pepper to taste. Serve immediately.