Gary, I haven't done one of these, but I think the loose band is for reference so you know how big to cut the tennon for the CB. I think it will fit over the middle bushing; once you turn the pen to size, slide that over the bushing and then turn a tennon in the upper ("cap") barrel so that ring fits. As for the other two, I can't tell from the pic, but does one of the bushings have a larger diameter than the other? If so, that's PROBABLY the one for the cap. One way to tell is to get out a micrometer and measure the diameter of both end bushings, and compare these with the diameter of one of the parts (cap/nib); one of the bushings should match the size of the part pretty closely, and the other should be off by a bit. If they are the same size, then I think they're basically interchangeable. I'd use the longer bushing farther away from the headstock (i.e. closest to the tailstock) in an attempt to add a bit more stability at that end.
Good luck!