Here is my procedure. My climate is very dry so left to their own many prints will curl until their corners touch.
1. I dry them between two clean screens. This has never left a mark. They come out flat with a bit of wave. This part is important as trying to press a seriously curly print often damages some emulsions by cracking it, especially the corners.
2. After they are dry I put them in a hot press and then pull it from the the press and let it cool under some books. The cooling part is important, because if the print is allowed to cool without being held flat, it will have a tendency to get back a little of its curl. By doing this they come out as flat as can be. On occasion if I don't mount them, they will curl back up very slightly. Mounting, or another session in the press cures that.