If you mean you are disembarking Sapphire Princess in Sydney for a flight back to the U.S., you collect your baggage on the pier, board a bus for the airport, check in and fly back. You go through Customs and Immigration on arrival back in the U.S..
Hope this is what you were asking
Oops! Just noticed this is your first post; welcome to cruise-chat--one big happy family with a common passion.