Hello GRIL232,
We just used our points on our Enchantment cruise in November.
Granduer Sailer is correct on the proceedure. You call and tell them you want to use your points. They will mail you the form/certificate, to fill out and mail back. They said it would take eight weeks for the form to get to me, but it was only about five weeks.
BTW, RCCL has the same policy that Samplecd2 described for Carnival. If you cash in your points on a cruise, they are NOT refundable, should you have to cancel that cruise.
Also, once you have them send the certificate to you, there is a time limit on it's use. I believe it is six months, (Unfortunately I don't remember whether that was supposed to mean you have only six months in which to submit the certificate, OR whether that meant you have six month in which to sail. But I think it was the former.)