The black bit isnt a Zender Extension. Mearly and uncolour coded piece of plastic, along with the bottom of the front bumper and the skirts.
I would reccomend getting the rear colour coded and fitting a pair of LE sideskirts and front lips or just colour coding all round, i.e what is allready there.
Leather as long as its from a Coupe, and has the frames attached is simply 4 bolts holding it down to the floor I believe. Move the seat back and forwards to get to them. Unbolt, lift seat out, new ones in and bolt them up properly. careful not to cross thread the holes though