I start the gravy when I pull my turkey out, so it's thick and ready to add the drippings after the turkey has rested a bit.
Melt the butter in a large saucepan and add the flour, stir together until smooth and cook over medium high heat. Let it start to darken, just slightly.
Then if you want a larger batch add the turkey gravy packet (I prefer McCormicks) and stir it smooth again and then add a cup of water.
Whisk and let it thicken up, a LOT. But make sure it's smooth.
Strain the turkey drippings into your saucepan, I just put a strainer over the saucepan, poke a hole in the disposable pan an tip it a bit, letting it all drain out.
Whisk it until it's smooth again and taste it. Add salt, pepper to taste, if it's a little strong (depending on the amt of drippings/juice you get) you can add broth or water even.