Watch this video from This Old House to learn how to install attic stairs.
1. Mark rough opening of new fold-down staircase on the ceiling joists in the attic.
2. Use a reciprocating saw to cut joists and nails holding any joists that must be removed.
3. Cut around ceiling trim with a utility knife.
4. Cut out the framing around existing hatchway opening with a reciprocating saw
5. Using the reciprocating saw, cut away all the old joists from within the new rough opening.
6. Cut new 2x8 joists and header to fit the staircase's rough opening.
7. Nail the side joists to the new header using 16d common nails.
8. Set the partially assembled framing into place; nail through the existing ceiling joist in the ends of the new side joists.
9. Fasten the opposite ends of the side joists with metal joist hangers.
10. With the new framing installed, use a reciprocating saw to cut the ceiling from within the new rough opening; collect the dust during sawing with a wet/dry vacuum.
11. Screw temporary 1x4 cleats to ceiling at each end of the opening; be sure the 2 1/2-in. screws go into solid framing.
12 Slip the collapsed staircase into the attic, then drop it down so that it rests on the cleats.
13. Tap shims around the staircase to center it in the opening.
14. Pull down the stairs and drill pilot holes through the side framing of the staircase and into the joists.
15. Fasten the staircase to the joists with 4-inch lag screws.
16. Remove the cleats and trim off the shims.
17. Pull down the stairs and use two 1x4s to determine the angle and length of the lower section of the stair stringers.
18. Trim the ends of the stair stringers to the proper length and angle with a circular saw.
19. Install casing, or other wood trim, to the ceiling around the staircase opening.
20. Paint the casing and staircase panel to match the ceiling.