Instructions
Trade friend codes with another player who is willing to trade you a Rayquaza if you are trading via Wi-Fi. You do not need friend codes for a direct DS-to-DS transfer.
Load your game and visit any Pokemon Center in any town.
Talk to the woman behind the desk on the second floor of the Pokemon Center to conduct the trade. You will be prompted to connect to a Wi-Fi connection or your friend's DS.
Follow the on-screen instructions to accept your friend's Rayquaza.