The basic procedure is well described in South Bend's "How to Run a Lathe" book (available through Lindsay Publications), L.H. Sparey's "The Amateur's Lathe," and other places. That will explain about setting your compound at 29 1/2 degrees, feeding with the compound so nearly all the cutting is done on one side, etc.
A major key to good threads is to have a SHARP threading tool. Then use an approprite lubricant. I use a high-sulphur cutting oil, slathered on. A lot depends on the kind of steel you're cutting, too. It's much easier to get good threads in Ledloy than in drill rod, for instance.
Once you've got the threads cut, you can clean them up a bit. first run a fine three-corner file along the thread in slowest backgear, if they're really rough. Then take a soft pine stick, dip the end in about #320 lapping compound, and press the end against the thread with the lathe running a couple hundred rpm. The soft pine will conform to the thread and track along it. Do that for a while, and it will polish up the thread pretty well.
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