Open the door until the knob touches the adjoining wall. Mark this spot with a pencil. Shut the door.
Drill a hole in the wall with a bit that's slightly smaller than the wall anchor provided with the bumper. Push the anchor firmly into the hole until it's seated flush with the wall.
Apply a thin layer of epoxy glue on the back of the bumper and on the corresponding area of the wall.
Place the threads of the bumper's back screw in the anchor, and turn the bumper to thread it into the anchor. Continue until the bumper is flush with the wall.