This is obtained in chapter 5.
At the beginning you enter the "main area" with no guards.
Go left down the cell blocks and you'll talk to a man who wants some sleeping pills in exchange for distracting of the guards.
After the conversation, turn round and go back to the main area but turn left twice and you'll end up in a hallway with the kitchen to your left and the pharmacy (where the pills are located) to your right..
The kitchen has one guard in it which can obviously see through the window and spot you if your in a position where he can see you.
There is only one other guard that patrols the pharmacy but there are two ways to get in..
Heather towards the main area again but on the left before you reach it is a door you can pick lock.. this route is easier because you can simply follow the guard that way instead of guessing his movements going the other way.
Plus you can avoid the kitchen guard easier this way.
Either way, once your in the pharmacy slowly creep towards the back where the pills are on a book shelf with 2 other items to grab here.
Head towards the door but get in the nook between the door and the table.. the guard cannot see you... once he goes, go back to the main area, to the cell inmate and voila achievement unlocks.
This is my 100% achievementguide and to see this getting done if you prefer visual to text.. go to 54:30 in my playthrough 1 video to see how and where its done.
Hope this helps!
Playthrough 1
Playthrough 2