if you r somewhere in the game , u fly to viridian city , then ask the old man who teaches how to catch pokemon ( bef0re that, put rare candy/ies in the 6th row of your bag. ) now, fly to cinnabar island, then surf to the east , then voila ! missingno appeared from your screen ( in battle )and u battle it (dont catch it because your game will corrupt. ) it sometimes appear in lvl 0, 80 ,160 or even 256! sometimes , you will also see mewtwo & clefable in lvl 146 , snorlax in lvl 150, electrode & chansey in lvl 165.