I fitted skirts to my zs tdci as I didn't have them from the factory. I had them painted by a local body shop.
There were two clips which push fitted into holes in the underside (you have to remove the two seals from the holes) There were also about four push in clips either side which fit into the sill holes on models which have them factory fitted (you can ignore these as you wont have the holes).
I then bought some automotive grade double sided sticky about 5mm wide and stuck this on the mating faces of the skirts at the top.
The holes on the underside of the skirts slid onto the fittings you push into the underside.
I then pulled a little of the backing tape off the double sided so you can push the skirt to the car and working from the middle then pull the backing off and push.
Then fit the end caps.
I never had any issues with them coming away and they were on for three years when I px it.