Hello! No, it's a good question, you're not being dumb at all :)
You start with your introductory Hazard, and every time you land on a new page, you should check the triggering conditions to see if the page you are on cause that Hazard to be in effect.
However, if you land on a page and its last digit is zero (and the page doesn't trigger an existing Hazard, like the introductory one), then you choose a NEW, additional, Hazard. The next time you land on a new page, you'll then check for the triggering conditions of both Hazards.
The intention is to escalate the sense of danger you're feeling as you play through the book - as you move on, you find new, plentiful, dangers around every corner which in turn (heh) make it feel like they're all out to get you, and the paranoia ramps up.
Note that you can't have two of the same variety of Hazard (so you can't have 2x "A Person is Following You") so the maximum number of Hazards is four.
Also, note that you can only trigger a specific Hazard (like "A Person is Following You") once per journal entry. So if you landed on a page and it triggered your first Hazard, and you skipped ahead the requisite pages to 'elude' the Hazard, and you landed on a page that would ALSO trigger your first Hazard, it does not happen twice. Any other Hazard could still trigger, so potentially you could trigger all four of the different Hazards for one journal entry!
Hope that helps!