Put the roaster directly on stove, add several tablespoons of the fat back into the roaster, (or sauce pan if roaster is not suitable for direct cooking), and add flour to make a roux, about equal to the fat, and mix with a whisk. Simmer, whisking the roux constantly while it cooks, the mixture will begin to thin and darken after several minutes. Slowly pour the separated drippings back into the roaster, less the fat, whisking the whole time. Also add the reserved liquid from the potatoes. If you need more liquid use more chicken broth. Simmer stirring with a whisk until the gravy is at the consistency you want. Taste before seasoning, you should not need much salt and pepper if any, it picks up the salts and seasonings from the roasted bird.