This is really NOT an easy task, and to be honest is probably best left unless you've got a real problem with the scratches.
The problem is when you polish glass, it produces a curvature which will distort your view through the glass. There have been plenty of people attempt this, and barely anyone has been successful.
My suggestion would be to leave it or replace the glass if it's a real problem.
I have a big curved double scratch on the drivers side of my windscreen - must have gotten some grit under the blade at some point (weird, has NEVER happened to me before) - any advances on PVR's solution or is that the only way of getting the scratch out ?
I have a couple of significant chips too, so toying with the idea of getting them filled under insurance - perhaps the windscreen guys can help with the scratch ?
It's fairly easy depending on how bad the scratches are.
I've used this kit over the years on my ITR and CTR and have had fantastic results. I've actually got to give the Z a going over as I have a wiper scratch and a few grit scratches on the drivers window.
My advice would be to mask off around the glass you want to do, cover the car with dust sheets maybe just so you have the glass exposed.
You will need a small pot of water to dip the pad in now and again.
You will also need a power drill, I doubt a battery drill will last for the job.
I used to think there was no way you could get the scratches out of a windscreen but these kits are brilliant. They will leave your screen with a brilliant shine. I used to do it twice a year on my ITR just to remove grime etc.