The decorator ordered inserts that were 16" diameter, and guess what, there is no pattern available for a ball pillow that size- so I had to draft one.
If you look at math websites, or, alternatively, globe websites, you can find how to draw the pattern, but unfortunately they don't give precise proportions for the curves.
I figured I could use the insert for a rudimentary pattern, and refine it from there.
The softest, most flexible material I had readily available was- Kleenex tissues!
Pinned to the pillow, they draped around the sphere, and were easy enough to see through to mark the seamlines.