I measured and masked, then painted a large piece of Velin Arches,
which looked like this:
For the shaped books, I did not fold the initial horizontal line, but
I did measure and mark it, made my curvy cuts, then erased the marks.
Scored across the connecting 1/2" bones, then folded up.
Some of the books have slightly different measurements to
allow for stems or to create the layered fishbone look.