Collect Silver Feathers dropped by Lugia flying overhead, then befriend all three Legendary Birds in Pallet Town.

1. Go to Pallet Town and build the three kits: Articuno (Freezing Chamber), Zapdos (Power Plant), and Moltres (Altar of Flame).
2. Interact with all three Legendary Birds to unlock the Tidal Bell recipe.
3. Collect Silver Feathers dropped by Lugia when it flies overhead.
4. Craft the Tidal Bell using 5× Rare Pokémetal and 5× Silver Feathers.
5. Wait for cloudy or rainy weather, place the Tidal Bell on the ground and ring it when Lugia flies over to register it.