Instructions
Insert "Pokemon Pearl" into your Nintendo DS's cartridge slot. Insert your "Sapphire," "Emerald" or "Ruby" game into the DS's GBA cartridge slot. Turn on your Nintendo DS.
Choose "Pokemon Pearl" from the DS's main menu. In the game's main menu, choose the option that says "Migrate from Pokemon Sapphire, Emerald or Ruby." Whichever game you have inserted in the game cartridge slot will appear as the option to choose in the menu. This will link the two Pokemon games so that the Pokemon can migrate.
Choose which Pokemon in "Sapphire," "Emerald" or "Ruby" you want to migrate; you can choose up to six Pokemon. Once you have selected them, choose "Migrate." Start your "Pokemon Pearl" game.
Go to the PalPark in "Pokemon Pearl." You will be given special Pokeballs that only work in the PalPark. Walk into the tall grass and walk around until you encounter the Pokemon you migrated into the game. Catch them with the PalPark Pokeballs.