Spoon off all excess fat from the top of the turkey stock. Strain the stock, in order to remove scraps left from the roasting procedure. Place the stock in a medium-sized saucepan. Mix 1/4 cup cornstarch with 1 cup cold water, and add to turkey stock. Bring to a boil over low heat, stirring constantly. Continue to boil and stir until gravy is thickened. When heating or cooking your gravy, you may find that it is too thick or too thin. To thin the gravy down, add water, a little at a time, and stir. To thicken gravy, mix some cornstarch with a small amount of cold water, place a few tablespoons of hot gravy into the cornstarch/water mixture and stir quickly. Repeat this step a couple of times, and then add the mixture to the gravy. It should thicken up without lumps. When gravy is done, taste it for salt, and add salt, if needed. The turkey stock is usually plenty salty. While hot, immediately pour into a gravy boat and serve. Yum!