Instructions
Find and battle all the Sinnoh region Pokemon in order to complete your Pokex. Encountering each Pokemon will allow the Pokedex to record the details about each creature, therefore you do not have to catch the Pokemon to complete the Pokedex. If you are unsure where the Pokemon are located, use an online strategy guide or purchase one from your local gaming store.
Go to Route 221 using the "Fly" ability of one of your Pokemon. Professor Oak will give you the National Pokedex, which includes the map for Pal Park. He will take you to do entrance of Pal Park and bid you farewell.
Walk into the entrance of Pal Park and speak with the park ranger standing by the door. For a small fee, you can enter the "Catching Show" tournament and receive six "Park Balls." The Park Balls allow you to catch and keep the Pokemon you find in Pal Park. Once you have used all of your six Park Balls, you will be transported back to the front entrance so use the balls wisely to catch the Celebi.
Walk to the north-west area of the park, which is the area where the wild Celibi roam.
Explore the wild grass area until you encounter Celebi. Although other Pokemon may exist in the same grass area, save your Park Balls for Celebi by clicking the "Run" option to flee from other Pokemon.
Use your Park Ball to capture the wild Celebi. Since the Park Ball has 100-percent success rate, you are guaranteed to catch Celebi if you encounter it. Once you have captured Celebi, it will be send to Bill's PC, which can be accessed through the computer in any Pokemon Center.