This rule gets skipped over all the time, in my experience. on page 52, in the description of the Cover status, it says "Attacking from Cover requires an extra Minor Action in order to get clear enough to take a shot..."
This isn't mentioned in the list of Minor actions, probably because it's not an action in itself. It makes the Attack action more expensive. Attack costs 1 Major and 1 Minor if you are in Cover.
Smartgun systems (page 260) include a camera that you can use to "fire from cover, suffering no Attack test penalties while affected by the Cover status". It isn't explicitly stated, but I would include the extra Minor action cost one of the penalties that smartguns negate.
There's also the Edge Action Fire from Cover which allows you to ignore the Minor Action penalty, but strangely it requires you to be in Cover IV to perform, even though the Minor action penalty is present at all four cover levels. This might have been an oversight in the errata process - it makes no sense to only have the edge action available at Cover IV when all levels of Cover impose the penalty.