They sit inside the cap and the wiring holds the ring flush against the cap.. I have them in several machines and none have dislodged even with the bumpers firing at full capacity.
pinballcenter did say you can apply a dab of hot glue to hold them in but I have found this is not necessary.
Any of the most commonly shaped bumper caps (with the raised outer edge) will be fine: (which will cover just about all of the modern Bally/Williams machines)
http://www.rtbb.com.au/catalog/index.php?cPath=40_115_117&sort=2a&page=1&osCsid=2727c8a370e84ee68cd879efea684147Check the cap style on your machine if in doubt.
They look much better in real life - I snapped this pic very quickly while I was testing them to make sure they all worked.
