To frame the width of the opening, measure (Image 1) and cut the new replacement header to size and nail it to the joists (Image 2).
When framing the opening, be sure the corners are square. This will maintain an even gap around the frame for shimming.
The rough opening is about 4" too wide, so we’ll measure and mark a reference line where we want to frame it in (Image 3).
Put a 2" x 6" at the reference line and nail it to the headers.
Cut three support blocks. Do not cut off the ends of a board to make the 2" x 6" blocks. Rip the board 2" and then make some 6" cuts. This way the grain will be running vertically and the blocks won’t split when they’re nailed in (Image 1).
Now, tap the blocks into position and nail one in place at each end and one in the center (Image 2). When everything is square, this will make installing the stairs much easier.
Put in a 3/4" piece of plywood as a filler for the ceiling (Image 3).
To install the stairs, use two people to hold the staircase in the frame of the trapdoor and have another person go in the attic to attach the stairs.
Up in the attic, slide the joist hangers onto the rods one at a time, placing them at a 45 degree angle to span the frame corners. Then, thread the nuts down but don’t fully tighten them (Image 1).
After all the hangers are in place, make sure there’s an even gap all the way around the frame (Image 2).
Once everything is square, tighten it up (Image 3) and then let go of the staircase down below.
Secure the frame to the headers with lag screws (Image 1).
Drill pilot holes through the shims. This will keep the shims in place and also prevent the frame from bending. Cut off the excess shims flush with the top of the stairs (Image 2).