Hop through the window and grab [(1/4) BLADES] and [(1/2) ALCOHOL] from the nearby table, then move to the end of the room, hop through the window and quickly turn left; collect [(3/4) BINDING], then move through the window leading to the balcony and hide underneath the window. When the guard moves away, go to the other side of the balcony and wait for a guard to come near to strangle him. When the coast is clear, loot Ethan's dead body for [3 REVOLVER AMMO] and move inside the room across the hall (with the yellow suits). Wait here until one of the guards moves in the direction of the guard you just killed, then move behind him to strangle him stealthily as well. The other two guards will usually inspect this hall soon enough (presumably because they've heard you), so repeat the same tactics to kill the other two.
Stealth Approach Start of Chapter
The room next to the info desk contains [(3/4) RAG] in the toilet while the lab room at the end of the hall contains [5 SKILL PILLS] and [(1/4) BLADES]. Once you hop through the window of the lab room, four more fireflies spawn, so hop back to hide. Soon enough they'll take certain positions in the upcoming rooms, and you'll want to take a stealthy approach considering the force of their weapons. Besides, a firefight will result in more backup Fireflies, which is something you'll want to avoid. Strangle the nearby Firefly as he walks to the room across the hall, then strangle the next as you sneak around while he checks on his dead mate. Be sure to take out the other two stealthily as well; it saves you quite some ammunition.
Stealth Trick with Brick and Firefly Pendant Bryony Stewart Seume
In any case, The area just beyond the lab room contains [(1/2) BLADES], [SUGAR] and a [HEALTH CAN] on a table. Also be sure to check the room in the center of this area's section to find [(1/2) ALCOHOL], [(1/4) BLADES] and [(1/2) BINDING]. The small room behind the counter at the intersection contains [(1/4) EXPLOSIVE] on a desk.
Move through the West Wing hall and hide behind the counter to let three Firefly guards pass. You can also find [(3/4) ALCOHOL] and [(1/2) EXPLOSIVE] here. As soon as you enter the next area, a Firefly will come out of the doors up ahead and spot you. However, it *is* possible to immediately hurl a brick at him, wasting him with a couple of melee attacks, without drawing the attention of any other Fireflies. It's worth a shot, and if it doesn't work out you'll at least have killed him; take down any others after this.
The small office in this area contains [(1/2) BINDING] and a [(1/4) RAG]. You can also find [5 SKILL PILLS] on one of the chairs in the hall. Head through the doors the Firefly came out of and go upstairs. You can find the [SURGEON'S RECORDER] on the counter. Search the room to the right of the hall to find [MARLENE'S RECORDER 1], [(1/2) ALCOHOL], [(1/4) BLADES] , [(1/2) EXPLOSIVE] and [(1/2) BINDING].
There's a closed door right behind the counter that requires a Shiv to open, which contains [10 SKILL PILLS], a [FIREFLY PENDANT (BRYONY STEWART-SEUME -304)] [RAG], [(1/2) BLADES], a [LEAD PIPE], [3 REVOLVER AMMO], [2 SHOTGUN SHELLS] and [(1/2) EXPLOSIVE]. In the next area, search the surgery tent for [(1/2) ALCOHOL] and another item, along with [MARLENE'S JOURNAL].
Continue down the hall and sneak past the doorway to the other side whenever the Firefly guard in the hall around the corner is looking away; if done correctly you can barely sneak past. Head inside the nearby room and sneak past when the Firefly up ahead is looking away. But instead of killing him, hide behind the desk in this hall and wait for a second Firefly to come close; strangle him first, followed by the other one.
Now while it appears you can just sneak all the way to the back and take out all guys by approaching them from the back, this isn't actually possible. Two last Fireflies are hiding behind the room's last barricade and they'll always spot you if you enter the hall just like that (or try to strangle someone). It *is* however possible to take out the remaining guards by using arrow headshots (and starting with the guy in the back). This leaves only the last two guards, which you should be able to handle. You could even lure those out by entering the hall, or making a sound, then dispatching them with arrow headshots as well.
After going through the door, head inside the room to your right and search it to find [MARLENE'S RECORDER 2]. After this, move to the end of the hall and go through the door. Without spoiling anything, simply make your way to the end of the area; it's easy enough and you won't miss any items. After this, several scenes will play.