I can only give you an illustration from my experience. Go to your Casino Royale account and when you see the offer click redeem and follow the instructions There will follow a page that will ask certain questions such as ship, date of sail, etc. Once finished and submit the request, an e-mail will follow to see if accepted.
Ps. There is also a $ value as trade in for any sails. For instance, I wanted to upgrade from the interior to a balcony, but the pricing was not up to what I expected. The trade in value was $500 and I used it towards a February 8 night sail on Anthem and after the C&A balcony discount and the solo supplement it cost about $750.