Things You'll Need
Instructions
Stand outside the door to the Underground Path on Route 8. Save your game here, because you won't be able to while triggering the glitch.
Walk down one step and pause very quickly. If you do this correctly, you will pause before the Gambler sees you.
Fly or teleport to Cerulean City. The Gambler should see you just as you get away without challenging you. Now your start button will be disabled for the rest of the glitch.
Walk north across Nugget Bridge and then east until you find the Youngster.
Walk along the border as far away from the Youngster as you can. If you walk up to him, the game will lock up. You have to let him see you and run up to you.
Defeat the Youngster's Slowpoke, then fly or teleport to Lavender Town.
Walk west into Route 8. The menu will automatically pop up when you walk onto Route 8. When this happens, press B to exit the menu. Mew will pop up when you exit the menu.
Throw a pokeball to capture Mew. Any pokeball will do.