You can do this a few different ways.
1) With a rewire tool (you can buy them from Piero or find in in multiple places in the game) reverse a Wall of Light. The earliest you can do this in the second story mission (High Overseer Campbell) - there is a Wall of Light right up the stairs from the place where you get off the boat. Once the Wall is hacked, just catch the attention of a guard, let them follow you through the wall and BAM, they're dead, and POP, you get the cheevo.
2) The second possibility is to knock a guard unconscious (with a sleep bolt or choke them from behind in stealth mode) and throw/carry them through the wall. I have not personally tested this.