1/4 cup turkey drippings and fat (margarine added as needed, see below)
1/4 cup flour
1 quart broth (or as much as you will need)
When the turkey is done roasting, transfer it to a carving board and cover with tin foil. While the turkey rests, take 1/4 cup fat and drippings of the turkey (if you didn't get enough, add some margarine to make 1/4 cup worth) and pour them into a saucepan.
In a saucepan, whisk the fat with 1/4 cup of flour over medium heat for 3-4 minutes to cook the mixture through. When it starts to brown a little, drizzle 1 cup of broth into the mixture, whisking continuously to ensure no lumps form. Then whisk some more.
The gravy will keep for up to a week in a covered container. Reheat slowly, whisking occasionally.
Note: for added flavor, you can a little wine instead of some of the broth, or a couple of splashes of brandy. Or for a country-style-ish gravy, use a parev milk.