Any time you enter a heavily guarded area, (they show up on the map) it'll warn you that an alarm bell is near. Sneak around the guards (and/or kill them) and make your way to the alarm bell, as long as you are not in combat you can click
to sabotage the bell.
Rinse and repeat (I got this by the time I did my first assasination for the assassins) and voila, you've got it.
(Some areas have more than one bell, remember you only need 5)