It needs new door bottoms, sills removed & all the sticky residue removed aswell or the skirts/sills wont sit flush as they should, plus the small side trims removed where the skirts fix to the side of the car, also underneath you should seal up any gaps/holes where the seals fit against the underside or it will allow road crap to collect & start to rot the bodywork, the S3 has the threaded bolts welded on chassis already underneath to screw skirts too, non S3's dont have these so skiaflex or the like has to be used.
I have fitted some 3dr skirts recently & literally took several kilo's of ballast each side of road crap that has become trapped behind the front wheels in a small cavity as such, used an airline to remove it all, but I was astounded by how much had collected in that area, which also led to paint lifting under the existing trims.
