What you need is a Pokemon that can use Teleport or Fly, a Pokemon that knows Growl, every trainer beat on Nugget Bridge, 1 slot in your Party of Pokemon, and every Pokemon healed. What you need remaining is: The JR. Trainer that's by the bridge available to battle, and the Youngster with the Slowpoke. Now note: You should've beat the Hiker with the Onix first before using the glitch. Now, heal your Pokemon at Cerulean City's Pokemon Center. Go out to the Nugget Bridge. Get to the end and go left until you can walk down. DO NOT walk down. Save your game. Now, walk down. Press Start IMMEDIATELY. If you did this correct, you will notice the JR. Trainer in your range of sight. He will notice you if you get out of the menu. Go to your Pokemon that can use Teleport or Fly, and Fly to Cerulean City. The JR. Trainer should notice you, but you'll teleport/fly away. Now, you're back at Cerulean. You're Start button won't work. and you can't talk to anyone (except Pikachu). The reason why is because the game thinks you're fighting the JR. Trainer. Now, go to the Youngster with the Slowpoke. Get in his range of sight, and leave one space between you two, or else the game will freeze. Use your Pokemon that knows Growl and use Growl 6 times at the Slowpoke. This will make Mew Level 1. Now defeat the Slowpoke. After the battle, the Start button will function again. Use the Pokemon that knows Teleport/Fly to get to Cerulean City. Now, after arriving at Cerulean City, walk up to Nugget Bridge. As soon as you change locations, the Menu will mysteriously appear. DO NOT OPEN ANYTHING. Leave the Menu. You will randomly face a wild Pokemon. That Pokemon is Mew. Use a Pokemon to make Mew Sleep/Paralyzed. Then, use a good Pokeball (no Master Ball, save it for Mewtwo) to capture it. You should have Mew. Now, switch Mew to the first slot in your party. Then, battle a wild Pokemon. (Hint: Don't let the JR. Trainer notice you. He can still be useful for more glitches.) As soon as you send Mew out, swap him out for another Pokemon that can beat the wild Pokemon. Use it to defeat the wild Pokemon. If Mew didn't gain enough Exp, then it should be elevated to Lvl. 100. The reason why is because Lvl. 1 Pokemon weren't coded in the game, thus causing the game to freak out and give Mew an INSANE amount of Exp, which should've gotten him to Lvl. 200 or something, but filtered it to Lvl. 100. The bad thing is that Mew only knows Pound, so teach it TM moves. Enjoy!