there must be loads of threads on this.
When putting them on, most people struggle to get the clip underneath on correctly.
This is because you push or bang it against the underside of the skirt, which in turn flexes up against the underside of the car. Therefore the cover clip cant clip inside the skirt.
What you need to do is attach the top of the jack cover into the outside of the skirt so that its 'hanging'; then get your fingers between the skirt and underside of the car and get the hanging cover in the palm of your hand and squeeze the cover up and inwards against the skirt.
This means you keep the skirt away from the underside of the car and allows it to clip in easily.
(for some you may need to use a screwdriver or something similair to allow you to get your fingers between the skirt and car)
Its 90% technique