Set up your iron bed frame, attaching the side rails to the head and footboards. Depending on the style of your old iron bed, the side rails may hook on or need to be bolted on.
Install the slats, which are pieces of wood that run between the two side rails. Make your own slats if the old iron bed doesn't have its own. Create at least four or more. Measure the width between the interior of the two side rails and cut pieces of 1-by-4 boards to an appropriate length. Lay the slats between the side rails with even spacing between each one.
Place the mattress on top of the bed slats, which support and hold it in place.