The caps have to be put in from the back of the wheel before they are mounted. They just have a lip on them. I ended up going back to the chrome caps. I liked the black, but they just were me.
When i cut them down i just measured in about an inch and made a mark. Then i put the caps on a flat surface, face down. Next to the caps i stacked up a couple of books and put a sharpie on top so that it lined up with the mark. This way i could just turn each cap against the sharpie and it made a perfect line all the way around them. After that i put painter's tape around the caps, right on the line. This gave me a good hard edge to follow with my Dremmel and cut off wheel (actually a few wheels). Once they were cut down and drum sanded i took a pair of Channel Locks and just made a new lip all the way around. Then i was just a matter of installing them. Here is a comparison pic of the black caps.