Deane, if you purchased your airplane tickets through the cruise line, your transfers are included. When you arrive in Miami, go to the Baggage Claim area where representatives from Royal Caribbean will be hanging around, holding up a sign that says, "Royal Caribbean." Tell the rep what ship you're on. After you get your luggage off the conveyor belt, the rep will show you to the right bus that is going to Port Everglades in Fort Lauderdale.
When the ship returns to Ft. Lauderdale and you claim your luggage after the cruise, once you leave the Terminal, a Royal Caribbean rep outside the terminal will direct you to the next bus leaving for the Ft. Lauderdale Airport.
If you did not make your air arrangements through the cruise line, you can still buy transfers that will do the same thing. Make sure you buy transfers for arrival into Miami and departure from the Ft. Lauderdale Airport.