Instructions
Travel to the Slowpoke Well, which is a cave located east of Azalea Town.
Walk around the well until you encounter a Slowpoke. Slowpoke's encounter rate is 15 percent, so expect to find several of them throughout your exploration of the well.
During battle, lower Slowpoke's health with attacks and then capture it with a Pokeball.
Walk towards the northwest corner of the cave. You will find a boulder blocking your path, but you can move it with the Strength ability.
From the boulder, follow the path south until you find a body of water.
Use the Surf ability to cross the water and reach the next platform. Step onto the platform and descend the ladder.
In the basement, use Surf to reach the man in the northwest corner of the room. When you talk to him, he will give you a King's Rock.
Equip Slowpoke with the King's Rock item.
Exit the cave and go to the nearest Pokemon Center.
Use the Pokemon Center's trade service to trade Slowpoke and the King's Rock to another Pokemon game. After the trade, Slowpoke will evolve into Slowking.
Trade Slowking back to the original "SoulSilver" game.