In Red/Blue, fly to Viridian (the 2nd) City and save your game, making sure you have in your party a PKMN that can use FLY and one that can use SURF. Once you're ready, talk to the Old Man that shows you how to catch a Pokemon. Once the tutorial is over, FLY to Cinnabar Island. Walk immediately eastward (to the right) and go up to the water. SURF, then surf straight up and down the narrow coastline (the half-ground/half-water tiles). You'll encounter Missingno there, and the 6th item in your inventory will be increased by 128. Make sure the 6th slot in your items bag is something you'll want a lot of, like a rare TM or a Master Ball or Rare Candies.
Or `M, which evolves into Kangaskhan.