In the first picture that I posted above, you can see the factory trim ring and notice that the filler lid sits within the ring. When you grind or cut off the ring, that allows the entire assembly with the door to basically fall into the hole of the fender. You have to carefully recess the entire assembly and glue it in place. In the 2nd picture, the black glue around the edge of the assembly is 3M 8115 adhesive. It is almost permanent once dry. In that same picture, you can get an idea how the filler door will recess into the fender. It is pretty simple to do and you really only modify the trim ring by cutting it off. Everything else operates just like stock.
This is what the stock fender looks like with the assembly removed:
I had a little help with a fatter lip around my fiberglass fenders:
[This message has been edited by IMSA GT (edited 08-20-2013).]