Guys these Terrafirma style extended arches...

Whe placed on wing there looks to be an awful lot of metal underneath...instructions just say line em up and screw em on...but is this really the way you stick them on? Or is all that metal underneath usually cut out to shape?

Dont get me wrong they look the part...but when you look underneath them, your staring at all the metal wing arch and also the screws. If thats the way its done then so be it, but if not - please tell me.

Instructions say little at all about anything.

Not 100% I understand your explanation, but I think you mean that the extended arches don't fit to the contour of the metal arch, and instead sit above, leaving metal arch below the extended arches, potentially fouling on your (presumably wide) wheels.
I'd cut out only what you need to ensure no fouling, and then line up the arches to suit.

I hope this 'elps, and that I got the right end of the stick! :L

YES!!! Nail on the head sir!!!

Good job considering I barely understood my own explanation.

The tyres dont actually catch on the metal - they were catching on the old arches.

I just was looking at them after fitting and wondering if all that metal arches underneath should be cut away or not...but then it seemed that the arch cover wouldnt be able to be fitted (the inner part of old arches that I cut off and refitted).

Really just wondering if I'm doing it right!