Instructions
Obtain the Sapphire in the "Pokemon Sapphire" version of the game. To do this, you must get the Ruby from Mt. Ember and follow the quests given by Celio at the Pokemon Network center on One Island.
Save your game at any Pokemon Center and turn off the Game Boy.
Connect your Game Boy Advance to the other Game Boy Advance system. Make sure that the second Game Boy has either "Pokemon LeafGreen" or "Pokemon FireRed" in it. You will also need to catch Articuno in this second version of the game. This Legendary bird-type Pokemon can be found on Seafoam Island.
Turn on both systems once you have Articuno in the second game and are ready to trade and head to the second floor of the Pokemon Center. Talk to the lady at the far right, who deals with cable connections, and select "Trade Center" when she gives you the option.
Talk to the trade lady in the middle of the room and select which Pokemon to trade once both game signals are ready. Articuno should be the one selected to go into the "Sapphire" game.