The "stack falls" if one or more dice that are part of the stack fall from it. Note that a die isn't part of the stack until it rests on top of the stack and you are no longer touching the die. This means that if you drop the die you're trying to place before you're done, the stack hasn't fallen. YET.

The dice must be placed on top of Boomstacker.

You must still tap Boomstacker if it attacks or becomes tapped for some other reason. That's the fun part.

You must use actual dice, although the dice don't have to be six-sided. Using coins and calling them "two-sided dice" isn't allowed. We recommend against using four-sided dice for this.

If Boomstacker is a token, you must stack the dice on top of whatever object you're using to represent the token.