@FomoFeraligatr asks
this holo has a curve to it but no crease. Do I put a stack of books on it or what?
Jeff answers
I wouldn't start with pressure, heat, or moisture. I'd put it in a suitable sleeve and protective holder and keep it somewhere with stable, dry conditions. Foil cards can curve, but a curve and a crease aren't the same thing. If it won't fit safely or you're considering grading it, get someone experienced to look before trying to flatten it yourself.
— Jeff, infinitepulls.com