I glanced about the rule for armor degradation, which looks easy and fast and effective.
However, it seems to remain silent on shield. Actually, it's silent on armor layering.
How to know what piece of armor gets downgraded if the character is wearing several pieces of armor?

Maybe do a simple top layer first?
Or maybe bottom layer first.
or split evenly between the two?

Or all at once. No free lunch!

sabs is right, though: you have options. All layers, top, bottom, split, hell—choose one randomly…

Yeah well, I feel more comfortable when theres a rule to it. It seems fair with clear rules.
Bah, I'll figure out something.

Well if it's 1 point of degredation, go with the armor that covers more of the body.
If it's several points, split them as evenly as possible.

That's how I would run it. PPP/Shields/helmets would go first, then down to other armors.

PPP also exist in a version to wear hidden under clothes, and in this case would be damaged later.

True.

Could be all layers if GM wants to be really nasty with this rule (it is optional). Also keep in mind armor incumbrance would be based on the original armor value.

I start with the outer layers. Armor jacket before form fitting. I also check the pow vs the outer layer so Pow 9 shoots through the armor jacket reducing its armor while still doing stun damage because of the Form Fit underneath.

That sounds like the most accurate way to do it, but it might be a bit more extra calculations than most people want. And it still leaves open the question of things like PPP. With most of it being an armor of about 1, that means it'd all get degraded on the first shot, which wouldn't make alot of sense.