Clean them thoroughly with detergent then prep wash to remove any grease and contaminants.
You can't sand them so don't bother.
1st coat with plastic primer.
If you want them smooth and not textured you will need a few coats of primer filler and a lot of sanding.
Otherwise just hit them with a normal grey primer.
Reason being that most likely your paint has a primer undercoat that is a different colour. If you spray a base coat over the plastic primer the finish may be a different colour to the rest of the panels.
Then a few light base coats in the colour of your choice.
Then as many layers of clear that you want applied. The more the better.
If in doubt take the skirts to a professional