First you'll need to figure out when to put the bird in the oven. Figure cooking time at 20 minutes per pound. Then add two hours more--an hour for the turkey to come up to room temperature before you cook it, and another hour for it to rest when it comes out of the oven. (If you're cooking a whole turkey and a breast, figure the time for each. You may want to put one in the oven earlier than the other so they finish at the same time. They will need to go in separate bags and roasting pans. Make sure to use pans that will fit in your oven together.)
You'll be using the lowest rack in your oven to roast the bird, so remove the top rack for now. Preheat the oven to 350°. Remove the giblets from the turkey. There will likely be some in both the neck cavity and abdominal cavity. DO NOT SKIP THIS STEP!
Salt and pepper the cavity well, then put the quartered apple, orange, and onion in the cavity. Rub the turkey all over with the oil or butter.
Open the baking bag and add the flour. Hold the bag closed and shake to coat the interior with flour. Carefully slide the turkey into the bag horizontally. Put the bagged turkey in the roasting pan and close the bag with the tie provided. Using scissors or kitchen shears, cut three 2-inch slits in the bag to allow steam to escape. Roast in the preheated oven for the allotted time.
When the turkey is done, take it out of the oven, and open the bag very carefully. It's crucial that you don't spill the accumulated drippings, you need them for the gravy. Enlist an adult to help you. Put the turkey on a cutting board or platter, and loosely tent with foil. Pour the drippings in a large measuring cup. Let the turkey rest for an hour while you finish any sides in the oven.