You make it sound so easy+1
Once you have the "Upside-Down & Wrong-way-round" sorted out in your head a simple box with positioning rails is all you need. Keep your "wrong" scraps to use as marking-out tools.
To get any angle right (even if the walls aren't square to each other. just pencil the lines of the 2 scrap pieces on the ceiling, where they cross over is your exact "cut-to" point.
Offer up your actual pieces and mark off as per your ceiling mark. Cut, butter with adhesive, apply!
I'm using a chop saw to cut the angles, have to cut them upside down I found or they don't fit together,
These are different sizes tall and wide as well so another thing to remember
Two more cuts to do and only just enough coving to finish so took a break so I don't mess it up
New girlfriend coming round tonight so might attend to her and leave the coving