If a new hole needs to be cut for the trapdoor stairs, make sure there’s no electrical wiring or plumbing in the way. It’s best to orient the opening so the stairs will run parallel to the joists.
Make sure that there is enough head room to move things in and out.
Take the correct measurement from floor to ceiling, because the stairs do come in different sizes.
If there is already a rough opening, the hole may need to be adjusted to fit the stairs. Start by taking off the existing trim from the trapdoor.
The rough opening needs to be 1/2" longer by 1/2" wider than the stair unit, this will give a 1/4" all the way around for shimming and squaring.
Measure and mark the length for the stairs and snap some chalk lines.
Using a circular saw with a segmented masonry blade, set it to the depth of the plaster (about 1" thick) and cut through the plasterboard (Image 1). Go slowly.
Finish up the corners with a jab saw (Image 2).
Remove the old header and a section of the joist that is in the way of the rough opening (Image 1).
Make the first cut through the joist cutting 2" back from the rough opening to allow room for a new header.
Cut through the nails on either side of the old header, and take out the joist section and the header in one piece (Image 2).