you have to steam a particular side, lay the board on a flat and ensure the bend hollow is to the ground kinda like this ( <--- spun 90 degrees to the right. then steam the top and then put something heavy and flat on it, or brace it down. you will never get rid of the bend fully with ply sadly, but you can straighten it out enough that when cut down into smaller sections they will be flat enough to use effectively.
ply is pretty evil when it comes to bending it usually happends when its got damp or wet and sagged or got to hot and the glue lamination have been heated enough to sofften and then re dried pulling the lamination's back together in an un-uniformed way.
The best method is actually to put it into an oven and heat it up to 50 degrees for about 10 mins and sofften the glue again, put the board flat and brace it to hell as the glue re-dries,
Both methods are not guaranteed to work with ply, as its a lamination of some substandard materials within the core react differently.
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