i only modified the skirts to fit the car, i used the original a3 fenders a few others on here that also retrofitted s3 skirts on their cars used s3 wings then they only need to modify the rear cups of the skirts and not the front and both like i did.
top section of the skirt needs to be cut off, as the door sills on the s3 and a3 differ, and the rear blades you can glue/epoxy onto the body, or you can use the original metal rails (like i did) and just rivet (the rails) it onto to body and the blades will be able to slide on and off just like on the s3's.
you will also need to drill some holes on the underside of the chassis for the screws to hold the skirts into place