To be honest, I don't know what I'm doing when it comes to making gravy, but I am doing it anyway.
When I begin cooking the turkey, I toss the neck and the bag of "goodies" (giblets, liver?) into a pan of water, along with an onion and some celery. I simmer the concoction nearly as long as I cook the turkey. When the meat is tender, I remove it and cut it up as finely as I can. (I shred the neck meat from the bones with my fingers.) I set aside the meat and the pan of juices.
When I finish cooking the potatoes, instead of merely draining the potato water, I catch some of it in a jar. I then filter out the celery and onion from the giblet water and add that water to the potato water. I toss the tired veggies but save the broth/potato water mixture to use in making the gravy.
Once the turkey finishes cooking, I pour the juices from the pan--all of it, which is probably breaking the rules--into a large saucepan and bring it to a boil. While I am awaiting that boiling moment, I pour some of the broth/potato water into a measuring cup and add a quarter cup of flour. I stir it until it is smooth. Once the meat juices start boiling, I stir in the floured broth/potato water, then begin stirring the meat juices as it boils. (I also prepare another half cup or so of the broth/flour mixture to add in a few minutes, if I don't think the gravy is thick enough.) I cook the gravy, adding floured broth, until I am satisfied with the consistency.
(The leftover gravy inevitably congeals a bit, and so I merely spoon off the fat from the top before using it.)