Read the other posts but the key to keeping it straight is to
keep in under tension. When you twist the stock, it will tend to
shorten up. If you keep it the same length, it will be arrow
straight.

For a short run you could clamp one end in a vice, slide a peice
of pipe the same length as your required twisted section, slip on
a washer or two to get a tight fit on the bar, then twist it with
large tap holder. You might need to have an auxilary clamp
(Vice Grip or two) to keep the bar from sliding. 3/8" hot rolled
is pretty soft, you can twist it easily.

SteveB wrote:

I want to spiral some small square stock. Probably 1/4" and 3/8" dimension.
What is the easiest practical way to do this? Hold one end in a vise and
twist the other with a crescent? How do I keep the whole thing straight, or
do I straighten it later? I want to spiral sections along a longer piece
rather than spiral the whole length.

And when I do want to spiral the whole length, will it spiral equally along
the length?

Steve