Contrary to popular belief, good gravy is *not* "easy peasy." Care must be taken or you'll end up with a clotted mess of yuck and not creamy, artery-clogging deliciousness.
You'll need about 2-3 tablespoons of fat, ideally the pork fat from cooking your sausage or bacon. If using sausage, reserve one patty as a "sacrifice." Over medium heat, sprinkle in about 2 tablespoons of floor. Don't dump it all in at the same time, sprinkle it in slowly stirring with a whisk or wooden spoon. This will form your roux. If your roux is lumpy, your gravy will be, too. The consistency should be smooth, but not runny. Add flour to thicken the roux, add fat to thin it.
Once the roux is at the desired consistency, keep stirring until it darkens to a light brown. Then crank up the heat to medium-high and whisk in about 2 cups of milk. Stir constantly until the mixture comes to a boil. Crumble up the sacrificial patty and toss it into the mixture, salt and pepper to taste.
Don't worry if it seems a little thin at first. The longer you boil, the thicker the gravy will get, plus it will set up a little as it cools. Whatever you do, once you pour the milk DO NOT ADD FLOUR in an attempt to thicken up your gravy. That will just clump up and make little nuggets of ick. It is ok, however, to add a little more milk if your gravy is a bit too thick.