The basic bumper consists of four parts. They must be assembled and you will need some momentary contact switches, some small hardware, and a couple of small springs. The pictures should give you the information needed for basic assembly.
The fifth stl file, drill guide, is used to locate the two 1/8" (3mm) holes in the rear of the robot top plate to mount the bumper assembly. The actual bumper is 8" wide and only 1/16" inch thick. This makes it flexible enough that only one side can actuate if the contact is only on a single side. That was you have three potential conditions from each bumper; left contact, right contact, full contact. And can program the robot to deal with those three conditions.