When you get to the pier the porters will provide tags for you. The cruise lines are getting out of the sturdy luggage tag business, with e-docs being the way cruises are ticketed. The porters won't touch a bag unless it has a tag attached. Don't forget to have a pen handy!
Alternately I guess you could go to the Celebrity people in the baggage claim area and get tags too, but we just do it at the pier.