Screw two pieces of scrap wood to the underside of the ceiling at each end of the opening with a power screwdriver. Position the wood so that it catches about 1/2 inch of the attic door frame without interfering with the door opening.
Remove the old door frame from the attic opening. Use a crowbar from inside the attic. Have a helper stand on a step ladder below the closed attic door to help keep it from shifting away from the scrap lumber. Remove any nails with a hammer. Lift the old attic stairs up out of the opening. With the assistance of your helper, carefully walk the old door unit down the step ladder and out of the way.
Place the new attic door assembly inside the attic with assistance from your helper. Have your helper go back down to the step ladder while you position the new door into the opening by setting it on the two pieces of scrap boards.
Check the square of the frame in the opening using a tape measure. Measure across from one corner to the opposite corner on each side. Use shims if necessary to correct any alignment issues.
Secure the frame to the joists using the provided lag screws and a power screwdriver. Attach the screws at the points indicated in the manufacturer's instructions. Have your helper remove the scrap board from underneath the ceiling that was supporting the frame.
Push the door open from inside, but do not extend the ladder stairs. Climb out of the attic using a step ladder if necessary.
Extend the attic stairs except for the last stringer. Measure the distance between the floor and the second stringer. Mark the measurement on the last stringer and cut it off from the bottom, if necessary, using a hand saw. Repeat for each side of the stringer.
Install the door handle according to the manufacturer's instructions and paint or stain the ladder assembly.