Befriend all 3 legendary birds first. Craft Tidal Bell (5 Rare Pok’Metal + 5 Silver Feather), then ring it while flying in rainy weather. Tip: Feed Moss Snorlax astringent food (Aguav Berry) to increase feather drop rate; turn up volume to hear Ho-Oh/Lugia flying by; use Sunny Doll/Rainy Doll to change weather