Instructions
Make sure you have a GS Ball in your inventory, then head to the shrine in Azalea Town. Place the GS Ball in front of the shrine. This causes Celebi to leap out and attack.
Use a technique to put Celebi asleep. Bug Pokemon such as Butterfree can use sleep powder, and Ghost Pokemon such as Ghastly can use hypnosis. Use low-level physical attacks to eat away at Celibi's health. Do not use any move you would use in a battle against a Pokemon Gym Leader -- this could kill Celebi, and you have only one chance to catch him.
When Celebi's health reaches 10 percent or less, use a Fast Ball, a Great Ball or an Ultra Ball to capture him. Celebi can dodge regular PokeBalls with relative ease. You should not use a Master Ball: while some Pokemon can only be caught with a Master Ball, this is not true of Celebi.