Well, with sausage gravy, the key is to render the fat from the sausage and then remove the cooked sausage, keeping the fat in the pot. A roux is one part fat, one part flour. So, whatever amount of fat has been rendered, you need an equal amount of flour. So what I'll do is put a sausage or two in a sauce pot and cook them until they're done. Then, remove the sausage and hold onto it to put back in the gravy. You may want more sausage later on if you deem it appropriate. Also, you don't need a lot. If the fat just barely lines the bottom of the pot, you'll be good. Then, set the heat to medium and put in the flour. It doesn't have to be perfect, but do your best to eyeball it. It should be at the consistency of a runny paste. At that point, you whisk
constantly. That part's important. After a while, the roux will begin to change color; you want that. It needs to get to like a chocolate brown before it's ready. It'll stink a little, but that's okay. At that point, to deglaze, toss in your milk. It's not an exact science, but I'll typically put in like 2 cups or so of the milk. Also, the higher fat content, the thicker the gravy will be. Bring that to a boil and put in the chunks of sausage. When it gets to a boil, take it off the heat and give it a few minutes to congeal. At that point, it should get pretty thick. If however it doesn't, there are a few things you can do to remedy it.
First, like Hev said, toss in some corn starch and put it back on the heat some more. Adversely, if you have more time, you could bring it to a boil again, then reduce it to a simmer and let it simmer for a few minutes. That will evaporate the liquid while keeping the solids intact. If everything went well, you should have some awesome sausage gravy. Also, I throw in a ton of pepper because I loves me some pepper.