For this achievement you need to discover all the memories within the third flashback, this will occur during episode 3 and is triggered by speaking to Locke after you've spoken to Sayid about your laptop.
You need to photograph Lisa with the "Rico's Shop" sign above her, once you've successfully done this the flashback will turn to colour and a cutscene will play.
Once you're back in control of your character you need to locate these three items;
• Recorder (on the desk)
• Notes (on the desk)
• An article (on some boxes to the left of the desk)
* Credit to 360VideoGuides for the video