Thinking about a "Proper" install

It seems that at least once a week, someone comes into the ICE section asking on advice for what to do for a good setup. I hope this thread will perhaps clear up a few questions that a newbie to car audio may have.

I'm not an expert on car audio, I don't do it for a living, but I have been into it for a few years now and I hope what I've learnt can be passed on to others so they can get it right first time.