First - take a deep breath. . . Okay, feeling better? Relax, it's okay.
It doesn't matter whose name is on the certificate, you can give it to your next door neighbor if you want. Turn the certificate over and fill out the appropriate information. I'm not home so I can't look at one, but if memory serves me correctly it asks for Ship, Sailing date, Reservation/Booking No., Name and maybe address, can't remember. I'm assuming your family is booked under one reservation/booking number so that is how the credit will be applied to your account. Not by your name or your husband's.
Anyway, fill that out and make a copy of both sides of the certificate(s).
Mail them to the corporate office at NCL, attention Accounting Dept. (or your TA if you want to but they'll turn around and mail it to NCL so go ahead and send it direct).
Go to the Post Office and send it CERTIFIED, RETURN RECEIPT REQUESTED. Do NOT send it plain ol' surface mail. You can of course send if FedEx, UPS, whatever, but usually the PO is cheapest. Just make sure you send it by some trackable means so you will know when it arrived and who signed for it.
Our family motto has become "free is good" so . . . enjoy your trip!