Go here if you are unclear with my description: http://www.youtube.com/watchv=BIz1RAg25yM

Supplies:
-Gameboy (Color, Advance, Sp should be fine)
-Nintendo 64 and Pokemon Stadium 2
-GBA Transfer Pak

If you wanna be catious, do this on a game you don't wanna screw up.

Step 1: Catch a Sneasel
Step 2: Raise it to level 57 and teach it Beat Up. It must be the third move or else you'll get a different Pokemon.
Step 3: Now, rename your Sneasel CCCCCelebi, or, just put any 4 characters in front of Celebi, like the first name I put up, for example. You could even do a random name. But you still need 4 letters in front of you Pokemon's potential name.
Step 4: Make 4 good clones of Sneasel. Do this by depositing your Sneasel and changing the box. Turn off the game while it's saving, but actually hit the power a split second after the word "POWER."
Step 5: Now, get a bad clone. Do this by doing the same thing but on Pokemon Stadium 2's GBA Tower. But only make ONE bad clone. The clone must be at level 0 and have garbage symbols for a name.
Step 6: Withdraw the bad clone and give it to the daycare lady. After, take it back and its sprite should be mush. Move the bad clone to the top of your party.
Step 7: After, move another Pokemon to the top of your party. WARNING: Don't use a Pokemon you wanna keep.
Step 8: Raise the bad clone in the daycare center. It should be second in your party.
Step 9: Deposit the first Pokemon in the PC, followed by the 4 Sneasels. Remove the mails that they might be holding. As you deposit each one, the name should decrease a letter each time. With draw CCELEBI.
Step 10: Take the bad clone back from the daycare center.
Step 11: Send the Sneasels to the daycare center one at a time. Deposit CCELEBI first, and take it right back. In the video it turns into a Hypno. Next, give the Sneasel name CELEBI to the daycare lady and take it right back.
Step 12: Erase the moves that Celebi doesn't normally learn and level it up and you'll have a legit Celebi.
Step 13: Release the clones, including Hypno. The gameboy won't recognize the clones so you can release them on Pokemon Stadium 2 by doing the following: Go to Professor Oak's Center, choose your game, and go all the way to the right. Now you can release them. This prevents any further potential glitches.


Done! This Celebi will register in your Pokedex and won't screw up you game.

I also suggest watching the video because it shows you exactly what to do.