First, a stranger must do something bad, wrong, or otherwise morally repugnant to you. They might cut you off in traffic, spend 5 minutes ordering coffee, or take your parking spot. The offense usually occurs in an incidental way where you don’t directly interact with this person, though that isn’t a requirement.
On his website, which appears to be powered by excellent – yet frustratingly mysterious – software, Alex Micek suggests a thing to do.