In a nutshell:
Calculate the how far the Go gauge will protrude when headspace is correct.
Set the barrel up in your 4 jaw chuck, indicate it in until the bore is running true.
Run the lathe in slow back gear.
Install your reamer in a floating reamer holder.
Gently run the reamer into the barrel, using lots of cutting oil, drawing the reamer regularly and cleaning it and the barrel of chips. Ream until the Go gauge will almost seat to correct depth.
Clean the barrel and assemble it to the pistol. Try the gauge again.
Using the reamer in a tap wrench, and lots of cutting oil, gently ream by hand until the Go gauge is accepted when the action closes.
Check with the No Go gauge. If the pistol swallows it, you've reamed too deep, and the barrel is pooched.
Never reverse the direction of rotation of the reamer.
There are variations, but that is a general guide.
I suppose that the barrel could be grabbed in a padded vise, and the reamer turned by hand with a tap wrench. This can be a good way to produce an overiszed or out of round chamber.
