You must have a ditto, PP ups and other items to increase the ditto's PP.

Put ditto first in your pokemon party. Go to Cerulean City, back where there is a field near Mewtwo's cave. Find a Pidgey, and while beginning to battle, transform into it. The attacks should be Sand attack, gust, and quick attack. Using the select button, switch Gust with sand attack. Defeat the pidgey. The move transform should be replaced with a nameless move with a type called 'cool trainer'. Restore the PP with the PP up. It doesn't always hit, but it makes cool graphics when it does! You cannot restore the PP in a Pokemon center, so you have to have PP ups on hand!