The standard advice is to talk to Shop 4 Parts. They have a wealth of knowledge about Fiat/Alfa and are more than happy to help.
For example, when my Punto HGT cam belt failed - I would have missed important aspects of the job without advice from S4P.
The 100HP wheels arch extensions are fitted with plastic peg style clips that just push into place. To remove with minimal breakage, first remove the inner wheel arch liners. Now with some large square ended pliers squeeze the underside end of the clip to relax it's grip in the wheel arch. It should then knock through. They are quite brittle so expect breakages.
The front bumper splitter has self tapping screws at each end hidden under the wheel arch extension. I have not removed the side skirts but would expect a similar system.
When the wheels arch cover is off, remove the screw (if fitted) and look into the end of the side skirt with a flashlight. You should then be able to see where a small crowbar style trip removal tool can get at the first clip along the car's sill. DONT just pull the skirt as the plastic will crease permanently.
eBay search "fiat exterior trim clips" will find dozens of clip types you will have to match what's fitted to your car.