1): Make Sure You've Gotten ALL 120 Stars. (Otherwise You Won't Be able to Follow Step 3)
2): Go Out Into The kastilyo Grounds.
3): The kanyon That You Probably Would Have Seen While Playing Outside The kastilyo 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 tuktok of castle. Remember, Mario is a fast faller, so aim high.
6): Shoot Mario Out Of The Cannon.
7): If you managed to get to the tuktok of the roof, you will meet Yoshi. He will give you a special message from Shigeru Miyamoto to the player, give you 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, You Must Get All 150 Stars.