This is only one known method of carrying out the Celebi Egg glitch and successfully obtaining Celebi.
Requirement
The Egg - The player must get an Egg that will know Beat Up as its third move upon hatching. This can be done by training a male and a female Sneasel up to level 57. Both Sneasel need to have the same four moves in the same order, with Beat Up at position three. Because both parents have the same moveset, the offspring will share this moveset.
The bad clone - The player must get a bad clone, a clone with a name made of only question marks (or, in some cases, blank), its level will be 0 and its gender will be different than the prototype's. If that Pokémon doesn't fulfill all those requirements, then it isn't a "bad clone" and it cannot be used for the glitch. This can be done by Pokémon cloning. The Box that is used for cloning must not be totally filled up at any time (otherwise the Box is unable to produce bad clones). Getting a bad clone is a case of luck and occurs sometimes during the act of cloning. The chance of getting a bad clone is at maximum if the Box has 15 to 18 Pokémon. Another way to increase the player's chances is by turning the game off after he/she selects YES, before any of the "SAVING... DON'T TURN OFF THE POWER." text appears.
Last requirement - The player must get any five Pokémon (preferably weak, common or otherwise useless Pokémon). The easiest way to get them is to clone a Pokémon until there is a bad clone. Because getting a bad clone takes some tries, he or she will get five unimportant Pokémon as a positive side effect of the bad clone action.
Steps
Step 1 - The player must first deposit the Egg into a Box. The five other Pokémon and the bad clone have to be in the party.
Step 2 - The player must then go to the Day Care and ask the man to raise the bad clone. He or she should then take it back from him, and it will be a ?????.
Step 3 - Next, the player must put ????? at the top of his or her party and go to the PC.
Step 4 - Using the "Move Pokémon without mail" option, the player must move any Pokémon (preferably weak or common Pokémon) from a PC Box to the top of their party. Its name should become the name of its original Trainer. The player will now have 7 Pokémon in his or her party.
Step 5 - Next, the player must go back to the Day-Care Man and ask him to raise the ?????. Afterward, he or she should return to the PC.
Step 6 - Back at the PC, the player must deposit the first and second party Pokémon.
Step 7 - Using the "Move Pokémon without mail" option, the player must move the Egg from its PC Box to the top of his or her party.
Step 8 - Next, the player needs to deposit the next four party Pokémon, but not the Egg.
Step 9 - Subsequently, the player should withdraw any Pokémon that he or she didn't use for the glitch.
Step 10 - Having completed the set up, the player should walk or bike around until the Egg hatches into a Celebi. However, the Celebi is still unplayable.
Step 11- The Celebi will hatch at level 0. Due to how the game handles Pokémon that level up in battle, the player must allow the Day-Care Couple to raise the Celebi to at least level 2. After this, it works like any normal Celebi.