The sheet was too large to use as it was, and the ceiling has a curious shape because of the stairwell. This is what I had - although at that point I had already cut out some bits. It's a ceiling pattern from 1756, never executed.
The central square is unfortunately too large for this ceiling, but I may use it elsewhere. The main problem was that I couldn't just cut out a bit since it had to fit the ceiling and be symmetrical, and my first attempt to paste cutout bits direct on the ceiling were a complete failure. It might have worked if I still had flat surfaces, but at that time I didn't have this wonderful paper. Working upside down in a limited space is not an option. So what I had to do is assemble the whole ceiling on a piece of lining paper. Luckily, I had a template, which is the floor above, still unfinished.
And then came the fun of designing a new ceiling, using Adam's patterns, like a jigsaw puzzle.
I started with a large bit that was quite straightforward.
Looks simple, doesn't it? Next, I added an octagon. I had to cut a curve, to follow the pattern below.
Then I added a side frieze. The arrows show obvious faults that I had to deal with, and the picture on the right shows how.
Then it was just doing the same on the other side, fill some gaps, add trimmings. As you see in the picture, it grew dark meanwhile, and I had to switch on the light. Believe it or not, it took me most of the day.
The side flaps will go down the walls, instead of coving. This is why they are upside down against the main frieze.
I tested it with white tack, but I think I need daylight and a bit more energy to glue it on. Report to follow.