Oven bags are manufactured from a high-heat-resistant polyamide film, safe for cooking in the oven and microwave oven. The oven bag keeps the turkey surrounded in its natural juices, producing a moist turkey with more flavor. Cooking your 20-lb. turkey in an oven bag reduces your cooking time by about an hour and reduces your cleaning time, as the bag contains any spills and splatters that occur.
Set the temperature dial on 350 degrees Fahrenheit. Allow the oven to preheat.
Open the oven bag and pour 1 tbsp. of flour into it. Hold the empty bag closed while you shake it to coat the inside of the bag with the flour. This keeps the bag from bursting in the oven.
Put the oven bag into a roasting pan large enough to hold the 20-lb. turkey. Insert an oven-proof meat thermometer in the thickest part of the turkey's inner thigh and put the turkey in the oven bag.
Tie the oven bag closed with the nylon ties provided with the oven bag. Use twine to tie the bag closed if you misplace the ties. Make six 1/2-inch cuts into the top of the oven bag, above the turkey with kitchen shears or a sharp knife. Push the ends of the bag into the roasting pan.
Place the turkey in the oven. Cook the turkey for 3 to 3 1/2 hours at 350 degrees or until the meat thermometer that you inserted into the turkey reads 180 degrees Fahrenheit.