I just used packing tape. The zip tie is fine but then you have it hanging there on the return trip as they will give you different tags for disembarking.
I just folded the printed tag as directed and cut a piece of tape longer than the tag, stuck the tape on the front side of the tag, looped it around a luggage handle and pressed the ends of the tape together. It work fine. But as Dave pointed out, just stapling would probably work too unless you are flying and are putting the tags on before you fly. Note that Dave drove to the port in his experiment.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007