Um. Forgive this old woman for not being as swoof as you had hoped. Your instructions say:
“Select the ceiling, rt-click, Cut Opening . . .
Repeat for the floor. ”
so I “selected ceiling. right clicked, then picked “picking points. and made a red dashed square above the stairs to make a stairwell. So, I see the red “dashed” box above my stairs. Going to the next floor, I attempted to follow the same instructions and I got a dark blue area whereby I would pick flooring. I managed to get some looped carpeting to cover the floor. Now, I have carpeting across the whole floor instead of a cutout. Things are not going according to plan. 🙁
Please help and use just a few more keystrokes and explain “cut” for me.