As others have noted, completing a location to 100% requires that you collect all resource items and destroy all Panauan government property in the area. You need to do this in 15 locations to earn this achievement.
Finding the resource items is easy: look for the proximity indicator in your HUD (top left corner); the more bars you see, the closer the item. These items can be weapon parts, vehicle parts, armor parts, or money boxes. Just keep covering all of the ground within the location's area (if you see a % in your HUD, you're in a location) and keep watching the proximity indicator.
Finding the government property is a bit more difficult, as they are not identified in your HUD. However, when you aim at them, the building's name and health will show (with the chaos symbol, the scorpion). And, of course, they will have red and white paint, with the Panauan star on them. Just blast away as these until they explode.
Exceptions: Fuel shafts and pipelines both require a quick-time event to destroy, rather than shooting. Communication antennas require that you extend them (press Y at the three consoles that surround them) before you can shoot them.
Some general tips on getting 100% in a location:
* Come with a lot of firepower. Grenades are very effective; I also recommend upgrading the assault rifle or machine gun as soon as possible. Best case scenario: Bring an attack helicopter or Stonewall/Razorback (light tank).
* Get good at grappling. Not only are many items on top of buildings, but it is helpful to grapple out of danger. Sometimes fighting the spawns can be a losing battle, both in terms of health and time lost killing enemies.
* I prefer to destroy all property first. I get the resource items that are around/available, but I'd rather come back to an item with a beacon than skip a piece of government property and forget where it was.
* Note that SAMs do not appear to contribute to your location completion %. Faction items also do not count.
Some tips on choosing locations:
* If you're just working on this achievement at the moment, always focus on villages (locations with a house icon). Some of these locations can have very few targets; I've visited a few that only had 2 or 3 resource items to reach 100%. The biggest problem you'll usually have in a village is a propaganda trailer.
* Avoid cities. These are incredibly rich in targets, and they're not always in the most obvious locations. Don't waste your time finding the last water tower on a small building surrounded by larger buildings, a resource item on top of skyscraper, or a propaganda trailer in a park.
* Military bases and naval bases can be worth it. Yes, more targets and more enemies, but most of these locations also have military hardware that make your job easier. You can do serious damage -- fast -- with a mounted gun you've removed from its base.
* Best areas for villages: Archipelago in the northeast; central coast area; south-central delta area; southwest desert; eastern jungles.
While working on this achievement, you'll make progress towards these achievements: