If you want to keep your documents safe, you'll need to install a lock on your metal filing cabinet. And they can be purchased at any home improvement store. The top drawer has a knockout, which is an oval-shaped cutout that you can remove by taping the center with a punch hammer. You can insert the lock cylinder into the hole, but make sure the bolt is facing upwards. Once the lock cylinder is in place, insert the key and turn it clockwise so the retaining clip is over the cylinder flush. Then, put the linkage piece into the bolt’s hole to lock it in place.