How do you activate the Vanish Cap in Super Mario 64?


Donkey Kong
Sorry if I sound noobish right now, but I just started playing Super Mario 64 again after a long break from it and am stuck on the level "Eye To Eye In The Secret Room". The blue box that I get the Vanish Cap out of is invisible and it won't let me hit it. How do I activate it?
First of all, lower the water outside.
Then, get in the hole.
Next, follow this video:
And finally, press the blue button.
I remember you have to beat Bowser first. Afterwards, use his key to gain access to the basement. Go through one of the doors until you see a room completely filled with water. There's 2 pillars. Ground pound them. The water will be drained. Once you're outside look for a square hole in the ground. And voila! You're in the Vanish Cap level.
Go inside, to the basement (the place with green walls and random fireballs on them), get the the door where you can see water, and jump into it.
Swim all the way to the other side, and you'll see 2 pillars; Butt Stomp (A + Z) on each of them, and the water'll disappear.
Now get through that metal door, and go to the right, until you see a hole.