Things Got Personal Achievement/Trophy
I did this once I found the first infected enemy early into the game. Once you grab the piece from the Bathroom on the second floor of the main house and escape jack.. again.. take it to the pillar in the room directly across from where you entered the main hall area of the house and align it so it makes a hawk. Now follow this path and you will come to a side room which can net you the "Open Your Eyes" Achievement/Trophy.
Here is a link for a guide on that one:
Now there is a door to the left of the area when you enter the room and once inside that room go through the next door to fight your first infected. They are pretty easy to kill so just circle around it knifing until you kill it. It will loose its arms after a few slices making it easier!
Here is a video guide for those having trouble.