Instructions
Turn on both Game Boy Advance systems. One should be loaded with a "Pokemon Sapphire" or "Ruby" cartridge and the other with a "Pokemon Emerald," "LeafGreen" or "FireRed" cartridge that contains a captured Ho-Oh game save file. A Game Boy Advance link cable should be connected to both systems.
Load the saved games on both systems. Make sure you have Ho-Oh as an active Pokemon in your party.
Enter any Pokemon Center on both devices and walk up to the desk with the emblem of two Game Boy Advance systems in front of it. Press "A" to talk to the person behind the desk. She will ask which service you wish to use. This must be done on both games.
Choose "Trade Center" and press "A" to confirm on both games.
Overwrite your saved game when prompted, on both games.
Press "A" when the screen reads "When all players are ready..."
Select Ho-Oh on "Pokemon Emerald," "LeafGreen" or "FireRed" and press "A." Select "Trade" and press "A."
Select any Pokemon on the "Pokemon Sapphire" or "Ruby" game and press "A." Select "Trade" and press "A."
Press "A" to confirm the trade when each game reads "Is this trade okay?" Trading will commence and you will now have a Ho-Oh on "Pokemon Sapphire" or "Ruby."