Here are instructions based on how I did the stem in the original post. My way involves very specialized skills, expensive tools, a high IQ, and better than average looks. Most people could probably never handle this at home. You kind of have the be the Steph Curry or Aaron Rodgers of the pipe world to pull this off. You average folks should find another, less interesting hobby.
First, find the crappiest, busted up and oxidized stem you can find. Pay top dollar for it if necessary so that yours is the worst around.
Next, put on some Bach or Mozart. Or Pink Floyd. David Gilmour will work too. A 33 RPM LP will yield the best results but is not absolutely necessary. But you should probably just not expect the repair to last as long.
Sand the stem all around the area to be repaired, and clean the stem inside and out. It should be a bit rough to provide "tooth adhesion."
Make some sort of dam thingy to keep your repair compound out of the airway. I used an old employee ID card that I cut to fit this particular area. Wax it with something like lip balm or coconut oil or something of that nature. Put a lot on. This keeps the repair compound from sticking to the dam. I used Decatur Pipe Wax.
Mix up some JB Weld. It it will be grey so you have to mix something in it to make it black. I used West System graphite powder because I have a bunch of it from another hobby and I know that a tiny bit provides
very black results. It also has good structural integrity and is scuff resistant. Mix it well. I thought about using epoxy and thickening it with the graphite but decided against it because more than 10% of the powder reduces the structural integrity, and JB Weld is essentially pre-thickened epoxy.
Slap it on liberally. Don't worry too much about shaping it at this point.
Put it down and try to forget about it for a couple hours. This is where the Bach or Pink Floyd comes in. Just chill a while.
After a bit, the goop starts to stiffen up and you can begin shaping it with a specialized, high-end titanium stem forming tool. Or a toothpick. With a little poking, and picking, and prodding, and rolling, you'll start to see it taking shape.
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Leave it be for several hours until it is fully hardened. Pull out your dam thingy. No, no, not your damn thingy, no-one wants to see that. The plastic thing you made to keep the goop out of the airway. If you lubricated it well, it'll pull right out. If you put it in dry, you're screwed. Take up knitting. Now you can begin to sand the repair to shape a bit more. I start with 220 Wet-or-Dry. Then mix a teeny-tiny amount of graphite powder (or pipe carbon dust, or whatever) into a couple drops of Super Glue. This is good for filling imperfections and low spots as well as sort of tying in the repair with the original vulcanite.
Flip it over and start on the other side.
After lots and lots of sanding with successively finer grit papers and polishing pads, and if you are talented, intelligent, good looking, and if God likes you, your stem will look like this.
Now you can pity the fool who takes the easy way out and just screws in another stem.
That's how she's done.