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.