How to Catch Missingno. in Pokémon Red and Blue

Make sure that your character's name is "ASH" without the quotation marks. Catching Missingno. depends on having certain letters in your name, and ASH is the easiest.

Go to Viridian City and talk to the old man located in the north. When he asks if you are in a hurry, say "No".

Watch the video the man shows you. The man will show you a video on how to catch Pokémon. After watching the demonstration, immediately Fly to Cinnabar Island. Flying to Fuchsia City also works.

Surf to the east coast of Cinnabar Island. Or, if you flew to Fuchsia City, surf south and west until you get to the Seafoam Islands.

Surf along the coastlines once you get to the Cinnabar Island.

After a while, you'll encounter Missingno. You may also see 'M, which works just as well.