So, as long as the dice are all touching and in more than one layer, they're stacked, right? Up to you in this case genuinely means up to you; if you have a lot of people playing and rolling, a single-column tower might be too precarious to last over six rounds, so figure out what works for your group.
Topple, then tally. Think of the toppling as a very messy way of rolling the dice that have been amassed across the rounds. And yes, you want the total of the numbers displayed.