Instructions
Beat "Golden Sun" and wait for the end credits to complete. The game will prompt you, asking if you want to save your adventure and warn that you cannot load from the data if you choose to save. If you want to keep your previous file, select another (possibly empty) slot and save the endgame data.
Wait for the game to restart and you will see the title screen with the "Press Start" prompt. Hold "Left" on the directional pad and the "R" shoulder button while pressing "Start." When the menu screen appears, look for an option on the right to "Send."
Select the data slot containing your endgame data, and then "Password." You will be asked if you want a "Bronze," "Silver" or "Gold" password. Select the Gold password to get the lengthy Gold Code for you to input into a copy of "Golden Sun: The Lost Age."