Instructions
Start the GBA game, such as "Pokemon Sapphire," and store Mew and Celebi along with four other Pokemon in your Storage System. Each time you use the Pal Park facility, you must transfer exactly six Pokemon. Save your game.
Put the GBA game and "Pokemon SoulSilver" into your Nintendo DS. Power on the DS system and go to the main menu of "Pokemon SoulSilver." Select "Migrate from [Game]." Choose the six Pokemon that you wish to migrate.
Go to the Pal Park facility in "Pokemon SoulSilver." Agree to play the game and you will receive six Park Balls. Look for Mew and Celebi in the forest of the Pal Park facility. Catch them with two Park Balls.
Capture the other four Pokemon to end the game. Retrieve the six transferred Pokemon, including Celebi and Mew, from the Pokemon Storage System at any Pokemon Center or the Pokemon League.