Bradrock this is the lathe part about reaming that I know - the barrel (either finished or unfinished) is chucked in the head. This is where a larger lathe comes in handy, because my mini-lathe simply isn't big enough for an unfinished barrel. Therefore I have to do the outside shaping first. A steady-rest helps.
The barrel is trued, then the tailstock is trued up to the bore. That *SHOULD* be a simple thing, but mine seems to be out of spec somehow. I can't get a good alignment on it. If anyone has suggestions, I'm willing to listen!
Anyway, you align the tail stock and that's where the reamer goes. You should mark the distance to the base of the cartridge on the reamer, like wrapping it up in tape. That's so you know if you've got the reamer in deep enough.
Chuck it in, use motor oil (according to the reamer rental site) or another thick cutting oil, and slowly work the reamer into the barrel. If everything is in alignment, you'll slowly, tediously, cut a chamber.
Now, I've read that you can hand-cut a chamber with a tap wrench, but I don't know how easy/difficult that is. It may be easier, I just don't know. I don't know if you just tap it "by hand" in a vise, or mount the barrel in a lathe and use your hand and the tap instead of the tail stock.
It would be interesting to hear other people's input on this. Perhaps a new thread... ???