the steps are below:
go to shoal cave(it shows up on the close-up view of the hoenn map)
talk to the man inside,he'll give yo instructions
Chat with our AI personalities
Every time the Pokemon holding the Shell Bell deals damage, the Shell Bell will heal the Pokemon.
You can find a Shell Bell at Hearthome City in Pokemon Platinum.
You can only catch spheals in the cave behind the city with the apace station, where you get the shell bell.
You find it somewhere in Mt Pyre.
You can find it at Hearthome City.