Spread them, and pop your thumb in the hole.
A 3D printed version of my wooden thumb page holders, available here, https://danatdandycrafts.etsy.com/uk/listing/1397820082/thumb-page-holder-page-spreader-book. Shape is identical, I just made two holes in the 3D model either side of the thumb hole. This was to reduce material use and reduce print time. Coincidentally, it makes them look really cool!
The centre hole is 25mm in diameter and are 12.5mm thick.
Designed by me and printed on my own printers (Elegoo Neptune 3 and Creality Ender 3)
As these are 3D printed, they aren't quite as smooth as my wood ones and may leave small marks on the pages.
Also available on Etsy here, https://danatdandycrafts.etsy.com/uk/listing/1825895954/thumb-page-holder-page-spreader-book, if that would be your preference.
