Keith you have to butcher the arches. Front is not bad because potentially you can always get new oem arches if you dont like it but its not the same with the rear. once you chop the rear its never going back. Im very tempted but this is what puts me off.
I looked into it. They are usually two types bond on or screw on. But I personally do not like either of them as I prefer smoothed look as it was always part of the car. The other thing is that most of the kits dont have cut off in front where the bumper meets the arch and if you need to remove the bumper you need to remove the arch extensions first. The other option is the wrc kit where you completely replace the front arches with wider one, but rear is still butchering job - ones done right could looks good.
So far from good bit of digging the only nice looking result with smoothed arches that I like is this one. And Im very tempted. Alot involved but I think it looks the business:
Full link with 30+ photos of the whole process here -
https://www.facebook.com/Dutchperformance/