Anyways, on with how to capture myuu, without the help of cheating devices (such as Gameshark).
Step 0: Make sure that you haven't killed everyone around Cerulean City (Misty's place), in particular north of there. If you have... well uhm... as of right now (as far as this walk through is concerned), you have 2 options: a) start a new game, or b) don't try thins cause it won't work.
Step 1: Ok! so, beef your pokemon up... but make sure that you only have 4 of them. If you have more than 4, deposit them in the PC. Stock up on pokeballs (6 or 7 at minimum... if you're not feeling lucky, go with 10). Have Joy heal your pokemon in the Cerulean (it's 3 in the morning so I can't tell if I spelled that right and I don't care), whether they are tired or not.
Step 2: Beat the 5 trainers, your rival, and 1 rocket (this isn't a very hard task if you have leveled up enough, even with only 4 pokemon in your party). Claim your Nugget and sell it if you want (it's not important what you do with the nugget). Feel free to go back down and heal your pokemon at the pokemon center AS LONG as it's still in Cerulean (why anyone would travel all the way to another pokemon center is beyond me). As a matter of fact, healing is highly recommended since you won't be able to do it again after this point. Save.
step 3: Head north of the bridge and then west. There is a skinny patch of grass there with a trainer in it. DO NOT fight the trainer... yet. Hunt through the grass until you find an Abra. Catch it (there are 2 ways to do this. First is by doing the Pokeball-to-Masterball code [press up and B at the same time as soon as you throw the pokeball and hold it, and your chances of catching the pokemon will be dramaticly increased]. Second is by putting the Abra to sleep using Sing (Clefairy or Jigglypuff) before it can teleport away and then weakening it and capturing it. I prefer the first method.). You now should have 5 pokemon in your party(Ref. pic: http://x2.putfile.com/8/23205425726.jpg ) Now, head north again till you're out of the grass and head east one space so that you're above where the trainer is but he's not on the screen. (Ref. pic: http://x2.putfile.com/8/23205442224.jpg ). Good idea to save here, cause this is your last chance to save durring this glitch.
Step 4: Really hope that you saved, cause this step will probably take you a few tries, even though it's really short. Press down to move south one space and press start right before Ash (or whatever you call your character) stops moving. If you did it right, the pause menu will come up ( http://x2.putfile.com/8/23205465452.jpg ). If you did it wrong, the trainer will walk forward and fight with you. If this happens, restart your game and load your savegame. Ok, after you did it right and got it looking like the screenshot there, click "POKEMON".
Step 5: Phew, the hard part is over. Now, go to Abra and Teleport ( http://x2.putfile.com/8/23205492653.jpg ). If you've been a good boy/girl and followed my directions above, this will take you to the Pokecenter in cerulean. But look at what happens before that! The trainer "spots you" (does the MGS
! thing) but doesn't come at you. You will teleport away before he can get a word in edgewise ( http://x2.putfile.com/8/23205513230.jpg )!
Step 6: Your start button is kinda broken right now. That's ok, you don't need to save now; as a matter of fact, you should not save untill you have Myuu in your party. Oh, and from this point on, DO NOT walk up to people and press A to talk to them. This will crash your game, or at least mess the backdoor process up and you'll have to start all over.
Ok, now head north and across the bridge again. Head east and follow the path shown in this picture ( http://x2.putfile.com/8/23205543536.jpg ). The guy to the left will fight you, and that's ok (if you want, you can battle this guy before you capture the Abra). Again I say, DO NOT walk up to him and get him to fight you by talking to him. Let him spot you. Now after you follow that path, you should find yourself one space left of this picture ( http://x2.putfile.com/8/23205564830.jpg ). Walk right and let the boy trainer spot you. Fight his Slopoke and win (not too hard with a level 23 Ivysaur like I had).
Step 7: Your start button is no longer broken. Don't save. Teleport using Abra again. Easy stuff
Step 8: Head north to the bridge again... What's this? ( http://x2.putfile.com/8/23206003468.jpg ) The game pauses itself?! wow. Do not save or access any menus. Instead, just press B to exit the menu and that will take you to...
Step 9: http://x2.putfile.com/8/23204342150.jpg
(Note: I was using Bad Game Boy for an emulator and PrtScrned that last pic, which is why it's bigger and has the graphic boarder. Oh, while I'm on that topic, this backdoor is a lot easier in emulators since you can use savestates at any point you want in the glitch.)
Step 10: Battling Mew can be a bit sticky, but not as hard as catching Abra. You can try the Masterball trick, but Mew won't teleport away as readily as Abra (actually, I don't think Mew has Teleport at this point). Mew is Level 7, so you might want a lower-level pokemon to fight for you, just so you don't overkill the poor thing. Putting it to sleep is helpful; Mew likes to dodge pokeballs, though it's not gonna dodge 99 out of 100 that you throw like some pokemon do. After weakening it to about 25% or lower, capturing it should be pretty straight forward. After that, save, and enjoy your new pokemon :3
Edit: OH! Abras aren't available that early in the game in version Yellow (obviously to keep players from finding the backdoor there). So if you're doing yellow, you're gonna have to wait untill you capture an Abra elsewhere or until you have a Pokemon with Fly (using Fly works as well as teleporting, as long as you remember to go to ceruleon and not any other city!).
....
