The easiest way is to cook it like a chicken – breast up, 20 min per pound plus 20 min. Even this very simple method does require a little preparation, though. A turkey is too big to roast in its own juices, as a chicken will with just a splash of oil. Before cooking a turkey, you should put it, ready stuffed, onto two large sheets of turkey tinfoil, one sideways and the other lengthways. Then smear lots of butter over the breast and legs – there should be quite a coat – and lay slices of fatty bacon criss-crossed over that. Then close up the tinfoil into a tent, leaving a hole for the steam. A general oven temperature of 180 degrees is fine, although it's best to give it half an hour at 220 first, to get it heated through. An organic turkey may take less time than this (2 ½ to 3 hours is recommended for a 12lb bird) and it will also cook slightly faster if there is no stuffing in the cavity. You need to baste the bird at least once.
When cooked, the juices will run clear if a skewer is stuck in the thickest part of the turkey. Allow to stand for 30 min before carving.