Depends on the Warband in question:
The more organized legions like the Word Bearers, Iron Warriors and Black Legion will most likely create a new society with them firmly on the top. Word Bearers will focus on the exaltation of the gods, so expect lots of statues, chapels and sacrifices to the gods. The population will most likely be forfeit, although those that pledge allegiance to the gods will be favored among servants.
Iron Warriors, as this user says, probably organize the world into some sort of fortress world, to ready for more raids and attacks on the Imperium at large, or simply to solidify their position. Fortresses, weapon manufacturing, and a focus on militarization would all be facets of the Iron Warrior's rule. As for the people on the planet, don't expect much in the way of mercy, but if you show worth or use beyond that of mere labor, I can't imagine the Iron Warriors would over look it.
Black Legion is probably the most likely to simply establish themselves as the leader of what ever was in pre-existence, if only because they were never the kind to take root in the first place. Rather, they rule through charisma, fear and strong central leadership. They know how to get the best results out of everyone under their heel; Have the Iron warrior run production, have the Emperor's child keep the nobles drugged up and in your palm, etc. And when a idiot steps up, or some one tries to out plot you, you step down hard on them as a example, just like Daddy Hor- I mean, Abaddon.
I imagine that Night Lords are very good at controlling planets through heavy use of psychology and fear tactics, although they would most likely install themselves as a presence like the Night Haunter; as in, give us tribute in the form of guns or bodies, or WE COME FOR YOU.
Who the fuck knows with the Alpha Legion? You might not even know anything has changed, until you get shot in the back
The big 4 warbands are all far too warped to do anything though