I did mine in a tub with warm water, vinegar and a splash of dish soap. It curled a bit on the sides, but I just moved it every 10-15 minutes so that every part of it had some time to soak. I turned it over and did the other side, too, so the curl had a chance to go both ways.
I then laid it flat on the floor and it was fine. It is still flat.
I think the key is warm water - not hot or cold - and moving it so that the curl goes both ways and it all has a chance to soak.