Instructions
Catch a Slowpoke. Slowpoke can be found by fishing with the Super Rod in almost any body of water in the Johto region.
Find a King's Rock. Players receive a King's Rock for completing "Mixed Mode" at the Trainer Tower in the Sevii Islands. King's Rocks can occasionally be found on wild Slowpoke; one is also located in Sevault Canyon.
Give the King's Rock to Slowpoke.
Connect to a friend's Game Boy Advance using a game link cable or the wireless adapter that came with the game.
Trade Slowpoke. It will automatically evolve into Slowking once the trade is complete.