Befriend all 3 legendary beasts first. Craft Clear Bell (5 Rare Pok’Metal + 5 Rainbow Feather), then ring it while flying in sunny 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