I'm going to assume that the "pickup cap" is something like what you see around the pickup (such as a Les Paul). If this is the case then...the pickup should go in first and the holes on the sides should be threaded for screws. Next take the pickup cover and put the screws in. On the side on the inside of the guitar put the spring on the screw then place the cover on top of the pickup. Push down a bit so the screw bites the threads on the pickup. This will allow you to change the height.