14/09/2019: This achievement is gllitchy at the moment due to server issues or bugs in the game.
There are four fallen Condor sites total. One is unmissable because you'll pass through it during a required mission, The others will show up as question marks when you get close to them.
Two can be found in Act 2 - chapter 4
; one way up north and the other around the center of the map.
There is another crashed Condor with a safe in the North communication tower mission.
The fourth can be found in Act 3 - chapter 2 and 3
in the South East corner of the map, it's in the storm on the way to the Turntable station. Keep as much to the right as possible on the way and you'll run into it automatically. You can also go back after you finished chapter 2.
There is a good amount of enemies found at these locations and after you've killed all of them you have to open the save with Jack and loot the content to complete that location.
A tip from Sashamorning: wait until it says the objective is completed before leaving
Tip from TheLightMaster: The story related Condor in the Cosmonaut facility mission (main mission) is not required for this achievement. I can confirm because I got the achievement before I did this mission. The one on the way to the North comm tower is probably not needed either but you need the components so it can't hurt either.
Here is a great video explaining it all in detail by Maka91
, make sure to give it a like on his channel!