It depends, but since the most salient and common example would be Roman conquest of a neighboring tribe or kingdom, I will attempt to address that in brief.

Although one might expect conquest and occupation to proceed in a different manner varying from region to region, when it comes to Roman expansion we can actually ascertain a fairly clear pattern.

Firstly, Rome would find a justification for invading or conquering a rival or neighboring power. This justification could be specious or legitimate — it rarely affected the eventual outcome. Often an attack or an outrage perpetrated against Roman amici, or friends/clients, served as the initial casus belli.

Rome would raise legions and move into enemy territory, always seeking to engage the foe’s main strength in decisive battle. Often the initial rapid reaction forces would be composed of whatever mobile troops available in the region — Rome’s suppression of Andriskos’ uprising in the Fourth Macedonian War comes to mind. If the legions were immediately successful, occupation would begin. If the foe managed to defeat a Roman field army, which was certainly possible during the early stages of the conflict, another force would be mustered with rapidity. Keeping with our example of Andriskos, Rome’s initial attack under the praetor Thalna was comprehensively destroyed by Andriskos’ Macedonians — the first complete annihilation of a Roman army the Macedonians had managed. Unfortunately for Andriskos, it was also their last.

A second larger and better equipped force would then move in and crush resistance.

Next, prevailing and overarching political structures would be dismantled. Local communities would be granted autonomy, but the pre-existing states would be annihilated. See Rome’s destruction of Vercingetorix’ hegemony in Gaul or removal of the Greek Achaian League for examples.

Within one single generation of the initial conquest and occupation, it was quite common for one or two major revolts to crop up and gain steam within the first decades of Roman rule. Gaul saw isolated revolts in the ~30 odd years after Caesar’s conquest, as did Macedonia and especially Britain; Boudicca’s famous rising falls into this category.

As the years passed and these revolts were inevitably crushed, however, the Pax Romana’s advantages would become clear. The regions in question would become integrated into Rome’s money economy, which spanned the Mediterranean — the elites would learn Latin and acquire Roman citizenship. While Rome’s rule did not benefit all equally, archaeological evidence shows a marked rise in material prosperity across the conquered west in particular.

Thus the high Imperial Roman culture was born — and though the initial destruction caused by the legions’ brutal conquests was often severe, eventually the population would accommodate itself to Rome’s new order. This was only after the freedom fighters in each state were killed, defeated and displayed in Roman Triumphs, of course.

Source: Adrian Goldsworthy’s excellent book Pax Romana.

Side question,for the roman conquest, is there any comtenporary sources coming from the losing side?