Another inexact science. If you want exact, Google it.
Generally it goes like this. Start with the giblets: in a medium saucepan, place the celery florets, half of an onion studded with a couple of cloves, a carrot, scrubbed and broken into a couple of pieces, the giblets (we always added the liver, but some say it gives a bitter taste), the neck and cover with water. Simmer gently for 45-60 minutes or until the neck meat is tender and done. Reserve the giblets and neck, strain the liquid into a container. I chop up the liver, the neck meat and add to my stuffing. The cooking liquid is for the gravy.
Turkey drippings are really fatty, and you don’t need all of them otherwise you’ll be making VATS of gravy. So drain off all but about 1/2 cup. I save the rest just in case I want it. I place the turkey pan, with its drippings on a couple of burners, and start scraping and stirring while adding an equivalent amount of flour to the pan, start with about 1/2 cup. Stir, stir, mixing it in and letting this roux cook and brown (but not too fast–don’t have the heat too high). When all the flour is incorporated, start adding the giblet brew first, 1/2 cup at a time, mixing it in, adding more, and when you run out of that and if the gravy is still too thick, add chicken broth.
If you over-added and it’s soupy, don’t despair. Put 1/2 cup cold water into a Tupperware-style container, add 3-4 tablespoons cornstarch, and shake well. Add this a little at a time until you see the mixture thicken up. Note: some use flour in that water mix, but mine always is lumpy and then I have to strain it, which is doable. I just prefer the ease of the cornstarch.
Salt and pepper to taste and get someone to help you tilt the pan to pour it into the gravy boat.
Note: It’s optional to cut up the giblets and add it to the stuffing.