At some point during the first chapter, you will find yourself in a jewellers shop. Due to the variety of ways you can enter this area, I cannot give exact directions. However, once you are inside the shop, there are two masks to pick up. The first one (for me) was the fake one. Look for the room with the display cases and the shop-front windows. Dispose of the guards however you see fit and then face the door exiting to the street. The window display to your left contains the mask. Pick the lock on the middle drawer/cabinet and retrieve your first prize.
After picking that up, make your way down to the bottom floor. There should be a single civilian here. You can
alert him, so be careful.
Look around the room, you should see a painting you can interact with in one of the corners. If you enter the bottom floor from the street, it will be to your right. If you enter from the stairs, it will be in the far-left corner. Make sure to extinguish the two candles nearest the door to the street. There is one on the crate next to the door and one next to the picture. This should
prevent him from seeing you while you plunder his goods (it worked for me on master difficulty, anyway). If he's being alerted by you extinguishing the flame, make sure you do it while he is at the far end of his 'patrol'. I use this term loosely as he only moves between his two desks. You could actually knock him out if you're playing on a lower difficulty. The choice is yours!
After securing the area, remove the picture from the wall by finding the secret switch. For me, it was at the top-right side of the painting. I'm not sure if this is randomised, so if that doesn't work check the whole thing. You should be confronted by a safe with three dials. Click the spoiler tags below if you want any hints or click them all if you just want the answer.
You can pick up these masks in either order. I just happened to find the fake one, first.
Any questions? Leave a comment and I'll make sure to reply. Hope I've been helpful!