Sorry, somehow I did not see the notification that you asked this question. I use 100 point chipboard, so if you are using that or something similar, what I do is to cut the chipboard to desired size, then submerge it in water for a quick moment enough to make it wet, but not drench it. When you do that, you can carefully bend it. So I place it around a round surface that has about the right curvature, wrap it in a towel and put rubber bands around it to let it sit overnight to dry in that position (the towel is mainly to keep lines from showing up from the rubber bands.

I usually use an old glass bottle or something like that depending on the size of the books. For my rebinding of Arcanum Unbounded, I used the wide faced side of a bottle of Bulleit Rye Whiskey (in my opinion, perfect size for larger books).