After you remove the nut the locking handle should just slip out. It may require some penetrating oil to facilitate its removal. Chances are its just frozen over time and not having been disassembled before. There are square shoulders on the lock that corresponds with flats on the locking handle. The above ferrule goes in the trunk lid and door, the locking handles go through it. You need a tool to crimp the edges over and hold it in place.
You may need a key to rotate the mechanism as you need to extract the handle assembly. That foot needs to clear the latch otherwise if not lined up, it won't come out. A locksmith may need to remove the barrel. Fisher body manual, rear compartment lock is the same as Chevy and other GM. Chevy shop manual