This is a rather straightforward, though missable achievement that is awarded for the completion of a side mission during this DLC's story.
I only thought to make a guide as I experienced what I'm sure was a bug and was unable to complete the mission and thus achievement requirement until I reloaded an earlier save.
To start this side quest speak to the man outside of the Palisade Blade entrance and he will tell you about his dead wife and thank you for listening.
When you are infiltrating the Palisade Blade there will be a room on the third floor (note this guide and the game use the British floor numbering system in which the initial entry level is the 'Ground Floor' and second level up is the 'First Floor') that you will have to enter to complete Shadowchild's side quest by putting the figurine on the desk in that room (which grants you another achievement if you complete that). On the computer in this room is the first piece of evidence you need to gather.
On the top floor of the building, but before we head up the lift to the server area, there is an office room which it is a primary objective to enter. Once inside you need to hack the computer to progress anyway, but make sure you read all of the emails on this computer for the second piece of evidence.
You should by now have an additional side objective to search room 209. Once we are in the secure server area, there are various vault type areas. 209 is upstairs on the other side of the room to the one you need to go in to progress. 209 is filled with poison gas however and if you don't have the lung augmentation, you will obviously take damage and die if you remain in there too long. This was the problem for me as I hadn't bought the lung upgrade and with all the green gas obscuring the screen, I was unable to find what I was supposed to be looking for and so decided to progress on with the story and come back once I had more Praxis kits. The game does give you the opportunity to come back here briefly but when I found the Pocket Secretary after doing the part where Jensen goes into the NSN to save Pritchard, it was showing up as 4/4 for evidence but not letting me read the pocket secretary, complete the quest or let me have the achievement. The lung upgrade isn't 100% necessary here but makes it easier to spot; look around the left hand side of the back wall and you should see it, and to reiterate, do this part BEFORE entering the area that you have to upgrade your Pass-Port to enter. Credit to vcllist for the addition that the valve to turn off this gas is in the first room you enter upon leaving the elevator.
The last piece of evidence is unmissable as it's on a pocket secretary in the first room of the next area which you must acquire for Oshiro's voice sample. Once you gather the last piece of evidence the achievement should pop straight away.
Recommended augments for this DLC are max Glass Cloaking, max Battery, Lungs and either the first leg upgrade or the strength arm upgrade. If you end up with Praxis to spare, maybe upgrade the hacking to allow you to do higher levels but there are so many Multi-Tools and crafting materials to make more lying around if you explore thoroughly that hacking isn't necessary really.
Hope this helps.