Far Cry 2

When ever you accept a faction mission (missions from A.P.R. and U.F.L.L. are faction missions) when you leave the building or on your way to leave the building you will get a phone call from one of your buddies saying they want you to go meet them. Once you go and meet them just keep pressing "Y" when looking at them and then you will accept their mission. Now when looking at your map you will see a blue marker and a red marker. The Blue marker indicates where you must go to subvert the mission. The red marker indicates where you must go to just complete the mission. So basically all you need to do is always go to the blue marker's and do what your buddy asks you to do. If you complete the blue marker objective and there is still the red objective don't freak out, just go to red marker objective point and complete the mission. Just do this for all of the U.F.L.L. and A.P.R. faction missions in the Northern Territory to get this achievement.
NOTE: I beat the game and subverted all the missions, if this sounds complicated or hard, its not really.

Good Luck

As an addition to the great description from SecondHalonick I want to mention that if you get to the Bowa-Seka territory (with all missions in Leboa-Sako being completed) w/o achievement unlocked you have probably missed or accidentally switched from one of those "blue" (subverted) goals to the red (main) ones at some point.

BUT FEAR NOT. All completed subvert missions progress is stored anyway! So if you missed one then load some early save and replay (subvert) just that mission you've missed. You'll get achievement immediately.

UPDATE: If you didn't unlock the cheevo then you might have missed more than one. Keep doing those "subvert" type missions you could miss and you'll get the cheevo eventually. It's still better and faster than replaying whole game.

Here's a little bit more detailed info: I finished the game and I've got all the cheevos except this one. Next I grabbed a game guide and started to look for what subverted mission could I fail or miss. To my luck, I found a mission I never played. So, I loaded my early game progress save, replayed (don't forget to subvert it) ONLY that mission and voila -- the achievement popped.

Hope it will ease the pain for those of you who were going to replay whole game just to get this achievement. ;)

Welcome

Every time when you get a main mission from either UFLL or APR one of your buddies will call you to meet him in a safe house every time you got a call go to your buddy for two reasons, first this will make the mission alot easier and of course for the achievement but there is one exception if you got a mission that your buddy is going to die at the end of it then feel free to load an old save and refuse your buddy help at the beginning of the mission by going directly to the red marked objective because those missions are not counted toward the achievement and it's good idea to keep your buddy alive at least this is what I'm thinking, good luck.

Whenever you get a mission, simply wait to receive a phone call from your buddy to subvert it, and then keep heading to the blue objectives. The first two missions have multiple blue objectives, making them harder, while subsequent ones are easier. Just stay focused and you'll be able to complete your six subversions. Don't get thrown off by what the subversions tell you to do. Just keep it focused and don't give up. Treat your buddies like a Faction C. Generally, their approach is less violent and brutal. It's a more peaceful, slow, methodical approach. Again, you just want to keep it focused. The first objective will be to meet your buddy. The second one will be the main one, and typically the third one will be to meet back up with your buddy or protect them. Stay on it and you'll have it. Happy hunting!