Yet another: Since the question was about soldering, the simplest thing to discuss is "soldering a wire" - whether to another wire or to a terminal, the principle's the same.

You can get jackplugs with screw terminals and/or with solder terminals. If you're using the screw, it's still a good idea to "tin" the strands before you screw them down, and most of the "old hands" I've known consider it best to throw the screw away and solder them - regardless. IF you can find a decent crimp-on ring terminal small enough, I'd personally go that way, with the screw, but good ones small enough to go in a jack are hard to find.

The principal that the wire needs to be as clean as you can get it is the "governing" one, since the jack terminals usually are fairly clean. The individual wire strands are often "drawn" with a lube that has "always been there," and it can be pretty tough to get it off to the point that the solder will wet the joint, especially on an old cable.

The "twist them together" also applies: you should always try to "wrap" the wire "into or around" the terminal (or the mating wire) so that it will hold itself in place before you apply the solder - since otherwise it's almost impossible to avoid some movement while the solder is cooling, which will cause a cold (high resistance) joint.

Most cable breaks are at, or very near the terminals, so if you can figure out a magic way to decide which end is bad, you can usually make a 19.5 footer out of a 20 - or a 19 if you guess wrong when you cut the first jack off (assuming molded-on "factory" cables).

To first Guest - no, it won't stay in tune unless you've got a glass (or better, steel) bodied guitar. And the best solution for Spaw might be that "foam-in-place" insulation stuff, squirted into the soundhole till it bubbles up over the strings. Won't keep it in tune, but makes some of them sound a lot better. (And those plastic wire ties make good spoon capos too, as long as you get them tight around both spoons.)

John