Things You'll Need
Instructions
Fly yourself to Viridian city, and start walking north toward Route 2. On the outskirts of the city you’ll find an old man who, if you speak to him, will ask if you’re busy. If you answer no, the game will transition to a short tutorial on catching a Pokemon using a Poke Ball.
Watch the old man's tutorial, which will trigger the first string of game code that makes the glitch possible.
Travel directly from the outskirts of Viridian City to Cinnabar Island, using your flying Pokemon. It’s essential that you not travel anywhere else beforehand, because this may disable the glitch.
Open your inventory and, using Select, arrange your items so that the Master Ball is the sixth item in your inventory list. The identity of the first five items is unimportant.
Surf the west coast of Cinnabar where water meets shore until you encounter a Pokemon called a MissingNo or M'. Kill, or flee from any other Pokemon you encounter.
Capture, kill or flee from the MissingNo. Your actions here are unimportant; it's the encounter itself that matters.
Check your inventory, and you'll find that the Master Ball now has a ? in place of one of its digits, indicating that the number has increased beyond 99.