1): Make Sure You've Gotten ALL 120 Stars. (Otherwise u Won't Be able to Follow Step 3)
2): Go Out Into The kasteel Grounds.
3): The kanon That u Probably Would Have Seen While Playing Outside The kasteel Grounds but couldn't use (because it had a grating over its opening) will now be ready for use.
4): Get Into The Cannon
5): Aim for the roof. Don't aim for the very top. But aim at the little flat bit at the sides of the huge spire at the top, boven of castle. Remember, Mario is a fast faller, so aim high.
6): Shoot Mario Out Of The Cannon.
7): If u managed to get to the top, boven of the roof, u will meet Yoshi. He will give u a special message from Shigeru Miyamoto to the player, give u a hundred lives, and he'll make your third jump sparkly
Well. That Was 7 Steps On How To Meet Yoshi In Super Mario 64. REMEMBER: This Does NOT Work In Super Mario 64 DS, Because Yoshi Is A Playable Castle, And To Unlock The Cannon, u Must Get All 150 Stars.