I mean, we don't have to do that, as they don't said anything in the thread haha, but basically you """ban""" one stage, then I ban one, then you ban one more, then I ban one more, and the leftover stage is the one we play!
After that, the loser of the match chooses the next stage! But the winner can "ban" one stage. And the winner also has to say what character he'll be using next.
These are not specific rules for this tournaments but it's how it's done usually I guess?