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You must fly, if you walk or surf there, the glitch will not work. You must surf at only one 'square row' east from the land. Make sure it is a real Pokémon and not a glitch Pokémon, which may cause unwanted side effects.
Place Pokémon in your party that know Fly and Surf. Talk to the Old Man at the north of Viridian City and ask him to show you how to catch Pokémon. Fly to Cinnabar Island. Surf directly on the east coast of Cinnabar Island. Catch a Pokémon over level 100 when it appears. Make the Pokémon win a battle and it will revert to level 100.