Instructions
Work with one of your current Pokémon until it loves you. You can raise its happiness by taking it for walks, letting Daisy brush it, and winning battles.
Set your battle party up so the Pokémon that loves you is in the first position in your team, meaning it is deployed first in every fight.
Travel to Slateport city and stop by the Pokémon Fan Club office. You will find it on the western side of the city.
Speak with each person in the Pokémon fan club. One of the fan club members will give you a Soothe Bell if you have a Pokémon with high happiness in your top slot.