I got approximately 2 cups of drippings from a 14 lb turkey, however this will vary with the bird and the recipe you use. Once you have the drippings measured out of the pan, adjust the cornstarch and water to the correct ratio. You can do this by changing the amount in the servings section of the recipe below and the amount of other ingredients will change as well.
Remove your turkey from the roasting pan when it is done cooking. Place on a large cutting board and cover with tin foil to lock in the juices.
While the turkey is resting, you make the gravy! First pour all of the pan drippings into a gravy separator. Let it sit for a minute and you will see the fat separate and float up to the top layer.
Once it has separated, pour the gravy into an empty can or container. When you do this, it is the fat that is coming out and most of the gravy itself will stay in the pourer. Stop when most of the layer of fat has been removed. You can also just pour it all into a large measuring cup and scoop out the top fat layer but it takes longer.
If you do not want chunks in your gravy of bits of skin, etc, this is the time to strain the contents of your separator or measuring cup. Pour all of the remaining pan drippings into your roasting pan again.
Place the roasting pan over a large burner on medium-low heat. Combine the cornstarch and cold water in a small bowl. Whisk to combine. (You want to do this in a small bowl in order to keep it from clumping when added to the pan). Pour in the cornstarch mixture into the roasting pan and whisk.
Continue whisking until the gravy thickens. Season to taste with salt, pepper, or any other desired seasonings from your roasting recipe. If you need more gravy, add more chicken or turkey stock. If it is too thin, continue to cook down. If too thick, add more stock. Pour the finished gravy into a bowl or gravy boat and serve!