I need help finding someone to give me a gamertag on a memory card for the original xbox, to play this game offline.
It has instructions. I'd assume you need to have a modded Xbox to use it. Alternatively, you could just play the Dreamcast versions; those let you play the game offline normally.
Its a PPF patch that goes to the default.xbe of the game, it makes it so when selecting offline mode a fake profile will show up iirc. This does require a modded system. If you don't have a modded system then this thread here is of interest https://assemblergames.com/threads/phantasy-star-online-xbox-gamertag.49737/