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It's handled automatically, if you have an army in enemy territory it will loot, but i think the time it takes is variable. You can just wait with a small army until the place catches fire, then go to the next province. I don't know if army size or composition affects the time it takes to loot, however.
i'm not sure what you mean webbrave, but nearly every military action i've undergone has lead to looting; ie. those flames in a province which reduce production. Of course, you don't get any money or anything, it's in the soldiers pockets. And as far as reducing a fortress goes the manual says you can with an assault, but I don't recall that happening in any of my games either...
The thing with looting is that your enemy loses income for the province for one year. I agree it would be fun (and realistic I guess) if you got some money if looting a province, but it probably has something to do with game balance that you can't.