Once you're done, try resting the warped paper underneath something reaallly heavy for a couple of nights. It's worked for me...on different paper but you might as well give it a go! (I used my laptop and 3 standard A4 sketchbooks) :)
I’ve been trying to flatten it with some books I have after I finish working on it for the day, and leave it that way over night. It works for a bit but then when I add some more it curls and warps again aha;; I’ll find something really heavy and try to flatten it once I’m all done with it, like you suggested, maybe that’ll work better than what I’m doing;;; Thank you! ;u; <3