If it's a 156 veloce the side skirts will be standard sportpack, if it's a GTA they will be GTA skirts.
Assuming its a 156 veloce there are 7 or 8 8mm bolts underneath - you need drive on ramps ideally, but can get to them with a socket - need to lie on your back though. The skirts are pegged along the length by studs and the front and back there are slide-on clips. the top edge was fixed with double-sided tape.
On a car 10 or more years old the skirts start to peel away along the top edge and tiger-seal or sikaflex is good to reattach them. A damaged one will need replacing. Due to age the clips front and rear are most likely seized up, so once the bolts along the bottom are removed the skirt will have to be eased off with a chance the clips at the front and back will break. Chances are a replacement will be missing the clips for the same reason. Bolt the replacement skirt on and use strong adhesive to fix the top edge to the sill. I'm not a mechanic but this is one job I did on a previous 156.