In World Building Part One: The Longue Duree, I talked about taking the big picture view of your setting by shaping its long-term structures in broad strokes. Having the outlines of your setting is important. This ensures some level of consistency and makes for a coherent final product, but you also need to think about things at the granular level, where there is change and interaction between the various components of your setting.
First, zoom in. Take a section of you map, for example of small northern province of the Calecian Empire we were just developing, and create one or more regional maps detailing that location. As with the other maps, you will want to note your ideas and decisions down in a setting bible (you will also want to reference this document as you work so as not to introduce inconsistencies. Before you draw anything, look at your world map and consider your setting bible. What elements should be on that page? How are those elements influenced by the local geography? What natural lines of conflict exist there?
Remember an empire isn't just a broad swath of homogenous space. You can be sure it is a textured landscape of peoples, languages and cultures. Suppose you look at your map of the Calecian Empire and decide to focus on a province you have called Rhuna. Looking at your world maps, you see that halfing tribes inhabit the area. You also notice a swath of hills and mountainous terrain lines its eastern border. Before you even get to the elves, think about these details for a moment. The province is called Rhuna, perhaps that is because the halflings in this area speak a shared language called Rhunic. So establish that even though they are divided by tribal loyalties, they have a common language and probably a common culture.
Yu should at this point develop that culture. What are their religious beliefs and practices? What are their political structures? Are any of the tribes united? There are countless ways to answer these questions. For example, the Rhunic people are divided into seven tribes who occasionally war with one another and their neighbors over territorial or personal disputes. Each tribe is ruled by a council of elders, and once every year the elders from the different tribes meet at a grand council to resolve conflicts and address grievances. For the most part the tribes exist together peacefully, but they are ever-threatened by the hill people (who are also halflings and speak a dialect of Rhunic) from the eastern mountains/hills. Every so often these hill halflings descend from their pastoral lands and invade for food and supplies. The Rhunic halflings worship a fertility goddess named Sarva who requires an annual blood sacrifice to fertilize the soil. The answers can go on and on, but you can see the point.
Now that you have established roughly what was there before the Calecians came in, you need to plot out the conquest of the province and its impact. Maybe it wasn't a very violent conquest, the Rhunic people just want to go about their life peaceful and avoid tensions with the hill folk. A brilliant Calecian general offered them the empire's protection if they allowed it to set up colonies and collect tribute from the tribes. Of the seven tribes, five agreed and two resisted. Though outnumbered by the empire and it's Rhunic allies, these two tribes united under a charismatic leader named Dhunar. Eventually he was captured and killed, causing an end to open rebellion, but to this day a small resistance remains and draws inspiration from his memory.
Soon you will want to elaborate on life in Rhuna under Calecian rule, but first dwell on more mundane details: what resources are available in the area? what natural challenges do the people face (cold winters? dry summers?)? what is their daily life like? Do they have written language? What technology do they have? How did all these things impact their interaction with the Calecians?
Once you've established these details hone in on the present arrangement in the province. Maybe the Calecians initially only asked for tribute in exchange for protection, but eventually imposed their system of government on the halflings. So far the Calecians appear clever and diplomatic, perhaps they took a soft approach to assimilation and lowered the tribute of tribes that allowed Calecian "speaker" to govern their communities. This raises a question with the tribes that accepted the offer (and we shouldn't assume they all uniformly accept): what is the political role of the council versus the Caelician speaker? To avoid conflict, the Speaker is put in charge of all financial matters in the city, but the council is given a say on issues of law and has full control of the local religion. This may change overtime, but for now the effect is that the speaker holds weekly meetings with the council to decide legal matters (you will want to devise the precise system). Against this backdrop of assimilation, a new resistance is forming around a mysterious leader. They can't face the Calecians openly, so the resistance murders Caelician officials hoping this will drain their morale and unite the tribes against the occupiers. Since most communities have a sizable population of Calecian elves now (mostly descended from the original soldiers who conquered the land) there is ample opportunity for the resistance to make it's point.
This is just the beginning of course. Since this is a fantasy setting you need to consider other things like local monsters, the role of magic, etc.
After you have done all this, you need to focus on specifics. Map out and describe each community (remember there are two types: those under Calecian Rule and those under control of the councils), create all the major players in your area (the speakers, important council members, resistance leaders, etc), elaborate on local trade, design interesting locations to explore, etc. This is where you really bring things to life. The personalities and places you develop here will be endless fodder for adventure.
Now take it one more step. Take a single community and plan it out in as much detail as you can. You don't have to do all of them, just one to give you an idea of how the different communities work. Determine it's population, name, major structures (both physical and cultural), key inhabitants, resources, conflicts, sites of interest. Daily activities, etc. Imagine your PCs walk into the town; what do they see? who do they meet? where do they go to get answers? where can they sleep and eat?
Now create interesting relationships between these elements. Maybe Yorva the barrel maker wants to marry the fishmonger's daughter, but can't afford the dowry. Perhaps the local council of elders is narrowly divided in it's loyalties to the Calecians, creating two factions in the settlement. To protect themselves against the hostile faction, the twenty or so Calecian residents hire body guards (often times recruited from the hill tribes mentioned earlier). Every once in a while there is conflict and even bloodshed (usually on a small scale and involving ale). Slowly what emerges is a community on the edge, filled with local tension because of the divided loyalties in the council.
This is just a sampling of how to approach local design. The key is it helps clarify the big picture by bringing those larger structures into focus at the street level. It is one thing to say "here there is an empire", but to make it live you have to examine it's roots and bowels. Together with the Longue Duree, this approach should provide a solid foundation for any campaign.