You will get this after finishing the game alongside the Shoot the Moon (beat the final boss) achievement.
It is recommended that you play on Casual or Normal for your first playthrough, which makes popping this achievement very straightforward and not all that hard.
However if at any time you do get stuck or have trouble solving a puzzle, down below is a link to the IGN guide which I found very helpful. It has both text and video walkthroughs, as well as collectible locations if you are going for those.
http://www.ign.com/wikis/dead-space-3/Walkthrough