I've installed several pull-down stairs and the biggest helpful tip I received was to screw 4 metal straps to the 4 corners of the stair assembly. The straps should be about 1 ft long.
The straps are used to secure the assembly in place when you drop it into the roughed-in hole in the ceiling.
Of course, you would screw some wood strips across the opening so the ladder won't drop to the floor.
Once positioned, you simply bend the straps over the roughed in frame and secure them with nails.
Now you can have your trusty assistant remove the wood strips so that you can open the stairs and finish the installation.
Be sure your assistant is trusty or you'll be in the attic for a long time.