If I remember right I just stacked a few starting on the floor with a protective layer in-between so as to not scratch them (a piece of paper is fine). Then add a 5kg or whatever weight on top. After a few days I turned them over and did the same again.

After that most were flat, only one or two needed some more individual time. I imagine that if after that if they're still not flat them maybe heating them slightly in the oven or even just the sun outside might do the trick.