James,
Soak the operculum in a cup of water with a spoonful of liquid dish detergent
added.  Start with hot water right out of the tap.  You'll have to soak it
for some time.  Overnight or even 24 hours is not too long.  Once it gets
"soft" and flexible, dry it off with a paper towel or cloth.  Take two paper
towels and fold them up so they are 4 or 8 layers thick.  Sandwich the operc
between the two layers.  Lay it on a flat, hard surface like a tabletop, and
place a flat, heavy object (like a large book) on top of it to keep it flat.
Leave it until thoroughly dried (I'd recommend at least 24 hours), and it
will remain flat.  You can't rush this.  If you put the book on it before it
is completely flexible, it may crack (I have done so occasionally).  And if
you take it out before it is completely dry, it may recurl.  But I have
successfully flattened hundreds of opercs, large and small, using this
method.  Note - not all opercs are supposed to be flat.  Some, like many
Murex opercs for example, have a natural "curl".
Regards,
Paul M.