Mark the cutout hole from inside the attic by driving a nail through the drywall at the long ends of the hole. Go downstairs and use a drywall saw to cut out the opening between the studs and carefully remove the drywall.
Frame the opening by placing a 2 inch by 6 inch stud between the joists and nailing them at each end of the opening. Measure the frame carefully to ensure it matches the manufacturer's specifications. Steps capable of heavier loads require the end frames to double by nailing a second piece of lumber against the first.
Lift the stairs into the attic by turning them at a slight angle through the opening. Pull the stairs all the way into the attic. Nail a piece of 1 inch by 2 inch wood under each side of the opening to hold the pull down stairs in place while they are being secured.
Center the stairs by placing shims between the frames. Nail the stair frame to the rough frame following the manufacturers directions and remove the supports under the frame.
Open the stairs and measure from the upper section to the floor. Transfer the measurements to the lower section of the ladder and cut the legs so the ladder sits properly.
Paint the trim pieces and attach them to the stair opening with finishing screws. Open the ladder and decide what you'll haul up the stairs first.