How to Duplicate Items in Pokémon Red or Blue

Get the item you want to duplicate and put it in the sixth item slot of your bag/inventory.

Fly to Viridian City and walk north until you reach an old guy. Talk to him and when he asks you if you're in a hurry, say "No". He'll then show you how to catch Pokémon. Watch him do it.

Fly to Cinnabar Island and surf on the right side of the island until you find Missingno. (Just a bunch of glitchy-looking blocks).

Fight it and after you win, the item in the sixth slot will have a glitch box instead of a number.