I'm not sure about a soft focus look; what flashing does is compress contrast, while leaving sharpness alone. Is that what you mean? Soft focus is also possible, but you'd have to consider old tricks like smearing vaseline onto the enlarger lens or nick one of the wife's nylons.
Anyway, when preflashing, I just slide a diffusor (fancy name for a piece of milky plexiglass) under the lens and expose. Then expose again without the filter. That's all there is to it, really. You control the degree of compression by varying the flash exposure.
A second enlarger also works, and has some advantages - but also some drawbacks (takes up space, for instance). I just do it with one and a diffusor.
You can also change the color filtration for the flash exposure. That way you can do a kind of split toned effect with e.g. yellowish highlights and cooler shadows.