The side skirts are held in place with a strip of plastic that is riveted to the sill, the skirts clip in to place with this strip and you will also find holes under the skirts for rivets. The front arches are held with captive bolts that go through the wings. The rears are simply riveted round the underside of the arch, each arch front and rear is also held with a large self tapper accessed behind the removable flaps. If you want to you can use silicone sealer to seal the kit to the bodywork, but it's best to use the original ford sealer, still available from ford AFAIK.
HTH