Here's how I make a Thanksgiving turkey (mostly from the new Joy of Cooking). I buy two 16- to 18-pounders, rather than do one giant bird. These instructions are for a turkey up to about 18 pounds; if yours is bigger, you'll need to consult the book for adjustments to timing and butter amounts.
Remove the packets of giblets from both the main cavity and the neck cavity. [The first time I made a turkey, I didn't know there was a neck cavity, so I roasted the little plastic baggy full of giblets. Oh, yum.] Rinse the turkey well inside and out.
In a large container (like a stainless-steel lobster pot or a cooler, or a very strong plastic bag—although not a garbage bag, as these are not necessarily food-safe), dissolve salt (see below) in 2 gallons of water:
2 cups of table salt OR 3 cups of Morton's kosher salt OR 4 cups of Diamond kosher salt
Yes, it matters! Depending on the size and shape of the salt crystals, you will need more or less volume-wise. I've always used Morton's in the past, but lately I can find only Diamond.
Submerge the turkey in the brine. Cover and place in the extra fridge in your basement (or any very cool spot you can find) for 4–6 hours.
Remove the turkey from the brine and rinse thoroughly, inside and out. Pat dry. Place on a rack over a jelly-roll pan and refrigerate, uncovered, overnight.
Remove all but the lowest rack in your oven; set it to 325° (I use convection at 300°). Place a V-rack in your largest roasting pan.
Put these aromatics in the main cavity:
1 onion, peeled and quartered
1 carrot, peeled and cut into 1" chunks
1 small celery stalk, cut into 1" chunks
8 sprigs fresh thyme
Truss the bird as best you can (this means tying the ankles together with kitchen string and looping it around the elbows). Brush the turkey all over with 4 tablespoons melted unsalted butter. Place the turkey breast side down on the V-rack. Pour ¾ cup water into the roasting pan.
Roast the turkey breast side down for 2 hours, basting the back and legs once or twice with 2 tablespoons melted unsalted butter. Remove the turkey from the oven. Put on your Orka gloves (or grab a few wads of paper towels), grasp the turkey at both ends and turn it breast side up. Return the turkey to the oven and roast, basting once or twice with pan drippings, until an instant-read thermometer plunged into the thickest part of the thigh registers 175°–180°, about 30 minutes more. I always have trouble finding the right spot to insert the probe, so I can't offer any words of wisdom here. But, in general, don't trust the pop-up do-hickey that comes in the turkey (although I have to admit I do a little happy dance when it pops up just when I thought the turkey was done anyhow!). If the turkey approaches doneness before the breast has browned, increase the oven temperature to 400° for the last 5–10 minutes of roasting. Remove the turkey to a platter and let stand for 20–40 minutes before asking Uncle Kenny to carve it.
Then you need to make the gravy. I like to make a big batch of plain gravy to mix with the small amount of strong gravy I make from the pan drippings. You can never have enough gravy at Thanksgiving!
Remove the V-rack from the roasting pan. Carefully pour off the fat, but not the browned bits stuck to the bottom of the pan. (If there's still some juice in the pan, just do your best to skim the fat off the top—or pour it into a fat separator.) Put the roasting pan over two burners and turn the heat to medium. Pour in 2 cups reduced-sodium chicken broth (I usually use Swanson's). Bring to a simmer, while loosening the browned bits with a wooden spatula. Reduce heat to low and cook for 5 minutes.
In a little bowl, whisk 2 tablespoons water with 1½ tablespoons cornstarch to form a smooth paste. Whisking constantly, gently pour this mixture into the simmering broth, and cook for 1 minute. Strain out the browned bits and season with pepper if desired (it won't need any salt!). Mix with plain gravy—tasting as you go, since you might not need to add all of the concentrate. (If you don't want to mix it with plain gravy, then you should double the quantities of broth, water, and cornstarch given above and just use it as it. You just won't end up with as much gravy overall.)