I have to install the crown molding around a wall that has an octagonal shape so some of the corner are larger than 90 degrees

The angles are probably 22.5 degrees, if you purchase a angle finder or use trial and error. I imagine mitering the pieces and then butting them together ( make sure you glue them) Http://www.thefinishingcompany.net/

Michael, I understand the angles, but you haven't asked a question. But I'll guess and continue... Cutting the angles you need will not be any problem, since they will be about halfway between a 45 degree cut and a straight (that's zero or 90 degrees depending on how you look at it). But the other consideration is that inside corner cuts on trim are done much better if you cope them (this still involves using a miter saw.) Guessing a bit further, you might not know how to do that. Just Google - how to cope trim. There are lots of article and videos on this. Good luck, Al K