You get this trophy in the fifth chapter which is called: Full House.
In one of the very first rooms there is a crowbar. I left it in the room to make sure I didn't lose it.
After you clear the entire stage and get the message to go to your car, go back and pick up the crowbar.
Exit the house and go to the back of it and you'll see a manhole. Use the crowbar there. The mask is in the sewer which you enter.
Here's a video of what you need to do below.
I take no credit for the video.