The lock is held in place by a nut over a washer on the inside surface of the door. Remove nut and washer, put the plate over, align plate parallel to edge of door. Replace nut and washer and tighten in place. Drill small holes or simply drive thread-forming screws if the material allows. Short enough to not go through the outside surface of the door.
Keeps the curious from using a pocket knife blade to try and rotate the whole lock to see what's inside. Somebody hammers a screwdriver into the key slot and turns it with a wrench, something's gonna give. Them doors ain't wall safes.