A softproof ICC profile is generated by measuring the inks on the paper that you are using. There are 1000s of combinations in QTR and there is no way the author of that software is going to generate SoftProofs for all the possible combinations.
He does offer a tool called CreateICC - and you print the 21 or 51 step targets on your paper with the K7 curve for that paper and then dry it and measure it using a X-Rite EyeOne Spectrophotometer with X-Rite Measure Tool. Then save out the info as a txt file and drop that on Create ICC.
We made a few (just a few) and those are available here: http://www.piezography.com/PiezoPress/blog/calibration/piezography-soft-proofing/
If you have an EyeOne and older Mac OSX you can use Measure Tool. If you have Windows you can use Measure Tool. Some of the users on the QuadTone RIP Users Group may have produced workarounds for instruments like Color Munki, etc...
QTR support their own software - and I just gave you the basics of how it's done.
SoftProof ICCs are only as good as the calibration of your display. And there are varying levels of calibration available. Using Spyder on a Mac Display is quite poor in comparison to using an Eizo CG with its own calibration system. The biggest difference is that Spyder reduces the color output of a video board necessarily in order to simulate calibration of the display. The Eizo can actually be calibrated without the video board being compromised.
Then the SoftProof is really only relevant to comparing to a proof in a dimming SoftProof Viewing Booth to which the display is similarly calibrated.
So the use of SoftProof in and of itself does not guarantee better printmaking. It is just one part of the sum of a proper calibrated display, and a proper calibration standard viewing condition to which to compare it to.
The goal of the soft proof is to cut down the extreme black point and white point of the display to the same contrast of light reflecting off ink and paper. So the display looks like ink on paper rather than bright contrasty dazzling light. The color of the paper and the inks will be simulated onto the image on the display when it is softproofed. It's quite handy!
We use it with Eizo CG and NEC Spectraview displays in a standardized imaging room with GTI SoftView Viewing boxes. We're within 96% or so of dead match from print to display this way.