I don't have a Classic but the newer Kobo Touch low cost. When I did it, it was left hand side near the top if you are holding it in front of you. Main thing though is to use something plastic so you don't gouge it up or otherwise bust it.
I posted a link of the Touch low cost with the cover off. Maybe they haven't changed the case all that much or the clip holders.
https://www.mobileread.com/forums/sho...d.php?t=232385