Before installing the staircase, make sure you finished the edges of the staircase opening. You will not be able to reach in there and do this after installation. Remember that front facing edge, will be the last staircase step so it must be finished to match your staircase.
I dry fit the staircase and noticed that I ran short on my wallpaper placement. It didn't reach the very top of the staircase. I had to match and patch a segment in order to create the correct height. Keep this in mind for your build, so it doesn't happen to you. This shows the importance of dry fitting before committing to glue.
If you modified your staircase to create the closet, like me, depending on the flooring you plan on using inside of it, you will have to do some preparations before you install the staircase. I am using a tile paper flooring for the kitchen so I made a template of the flooring under the staircase, which will be the closet floor, before installing the staircase. This will make it easier to apply flooring in here later on, as the space is too small for my hand to go in easily and make the template later on. With the template, I will have a guide as to the size of tile paper I will need to apply in the closet. Any sheet flooring of this kind, will require a template. If the flooring is applied, plank by plank as in hardwood, a template will not be necessary.
Finally, I can install the staircase in place.
Apply glue, only to the floor edge and the top ceiling edge of the staircase. Do not apply glue to the side that will be against the wall. You need to adjust the staircases fit during installation and the application of glue to the side of the staircase might smear your wallpaper as you slide the staircase in place.
You want tabs to go into slots, as best as possible, but your priority for this installation, is for the staircase to fit flush against the wall. If you were careful during assembly to keep everything as square as possible, this should not be an issue. If the staircase has minor gaps along the floor, this is fine. It will be hidden with the baseboard trim but keep the staircase flush against the wall, regardless of the floor gaps. The floor gaps can be hidden but gaps between the wall and staircase can not.
