This process must be started no later than the day BEFORE Thanksgiving, in the morning.
Buy your turkey from a butcher or a place with a meat counter, and when you do, ask them for random, discarded turkey parts (backs & necks); they will always have some and you can buy them for very very cheap, like 70 cents/pound. Get about 3-4 pounds of that shit.
Take the turkey bones and put them in the biggest fucking pot you have; also, since you're probably going to be doing something with onions and celery, maybe carrots and some herbs, the next day, take the stems and the tops (clean them) of those things and throw them in, too. Make sure you get some onion skins in there, they will make everything a nice dark color. Fill the big ass pot up with water and put in a bunch of salt. I don't know, maybe a handful. Salt is important at this point because the heat and the salt and the protein from the turkey bones will make a natural MSG (or Umami; the 5th sense!)
Put it on the stove and cook, covered, on low, for no less than 8 hours. I'm serious. Don't cut corners here, even if Alton Brown says you don't have to cook stock for that long, fuck that guy, what does he know? You want the bones and the cartilage to break down enough, and I say cook it for 8 hours, dammit! If you have doubts about this part, read the title of this post again.
During cooking process, lift the lid and check things out every hour or so; pieces of turkey will float to the top and stick out of the water and you want to make sure everything stays wet the whole time (that's what she said). After 8 hours, strain it twice. Once to get all the big bones out and the next time to get the small pieces of crap that have fallen off in the cooking process; you don't want to eat that shit later. Use a fine strainer for part #2. Put the liquid in the pot that you plan to cook it in the next day and stick it in the fridge overnight.
Go to bed.
Happy Thanksgiving! In the morning, the liquid will be the consistency of Jello. This is what you want, it means that you cooked the shit out of the turkey bones and have created a fucking flavorfest in that pot. Go about your business and make your turkey and all the sides, you don't need to do anything with this for a little while. If you have some herbs you like, put the pot on the stove on very very low and throw the herbs in, if you want, it doesn't matter, but get it onto the stove, on low and boiling, at least an hour before your turkey comes out of the oven.
Cook your turkey on a rack so that the juices will drip down into the pan and you can collect them later. This is an important step because those turkey drippings and little crispy turkey pieces are really fucking delicious and you're going to want them later. When the turkey comes out of the oven, put it aside and collect all of this awesome shit at the bottom of the roasting pan and put them into your boiling stock. Let that do its thing while you bustle around and try to get your fucking family to leave your kitchen so you can cook in peace. I DON'T NEED ANY FUCKING HELP THANK YOU VERY MUCH.
Make a roux, which is equal amounts of butter and flour; it depends on how much liquid you have, but probably 1/4 cup each (half a stick of butter. it's just easier that way). Melt the butter, throw in the flour and then cook it, on low/medium. Alton Brown says that the lighter the roux, the better it will thicken the stock; the darker the roux, the more flavor it will have. Do whatever you want, I haven't ever found much of a difference. One thing I did learn today, though, is that you can make your roux at any time during the day on Thursday, which may be a good thing if you're like me and by the time you get to the serving part of Thanksgiving and you're drunk off your ass, making a roux is quite challenging. One year I had to make it three times because I kept fucking it up and burning it.
Anyway, make a roux, and then throw it in the pot. At this point, if you have anything chunky like herb stems or whatever, take those out. Let it cook for at least another 10 minutes so that the flour and butter can fully incorporate into the liquid. You will notice that it will also thicken considerably so stir often (if you remember; if not, whatever).
That's it. Serve the shit and then listen as everybody at your table says "Holy crap, this is the best fucking gravy I've ever had." Enjoy!!