If you chose the ladder you'll have to open the locker and grab a metal bar and examine a dead body while looking at the zip-ties and bullet. If you chose the ladder, you have a qte.
After that cut down the walker and look at the duffel bag, then open the locker when you can get control. Then try to open a door and the cash register, which you'll have a qte, then have a few choices before you get your achievement