Use a link cable to connect to Mario Golf: Toadstool Tour on a Gamecube. The game stats and tournament ranks, as well as any experience earned by shared characters in both games will be transferred to Mario Golf: Advance Tour.

Bowser: Have 81 Birdie Badges in Mario Golf: Toadstool Tour
Luigi: Transfer your experience in Mario Golf: Toadstool Tour
Waluigi: Have 27 Best Badges in Mario Golf: Toadstool Tour
Wario: Have 54 Birdie Badges in Mario Golf: Toadstool Tour