For this achievement you must never get detected in a level. If you do get detected you can simply restart the checkpoint, if you do get spotted quickly pause the game and press restart checkpoint Also if you never get detected in all the levels you get the Ghost! achievement and this can be done on one of the two playthroughs of the game that you have to do. one playthrough you must kill all the enemies and the second playthrough you must not kill a single enemy whichever one you start with try to add the Yurei medals (the detection medal) on one of the playthroughs.
*I recently wrote a guide for getting the first non killing medal you can check that out for some additional hints mainly with the two big hints regarding the last level where you get all the medals regardless and difficulties settings for the game