The RAW rules for shields as cover instead of using the Active Defense, Block, start on B407. That is, they're the same as the normal rules for cover. You don't Block with cover, and it matters only when the attack actually hits your cover instead of you. That's harder to arrange with a shield than, say, a wall, as people carrying most kinds of shield have most hit locations exposed while using it. But of course there were techniques to arrange more cover in groups (shield walls, testudo, etc) as well as just carting about really big shields up to the point where they've really become portable wall sections, like a pavise or mantlet.
People have certainly made up their own
house rules for shields-as-cover, but they're not really necessary. You might want to write down your own conventions for consistency or reference ahead of time, if that's the way your group rolls, or consult others' ideas (as with the link above) if you want to see how other people apply those rules, tweak them for extra granularity, or whatnot.