If the turkey is already cooked than you can just do it the way you mentioned. If you had gravy or turkey juice left, I would put a little in so it doesn't get too dry. All you would need to do is put it in the oven long enough for the pie to get good and hot and the pastry to brown. Depending on your oven 45 min to an hour at 350º. If you're looking for a recipe, try googling chicken pot pie. A different bird but basically the same.