I've heard there may be a couple of different fittings, but if yours is like mine (2004 Veloce) then it's straightforward.
The skirts are held on by some screws underneath, and another fixing ahead of the rear wheelarch (behind the circular, body coloured plug). The screws underneath are reasonably accessible, though it's easier if the car is jacked upon the side you're working on (observing the usual rules on safe lifting etc.). The one at the back, behind the plug, might be corroded to the point of snapping). Mine did. The others all came out ok. I cleaned them up and applied some grease and left them to one side. The skirts are held on (at the top, where you don't want a nasty gap - but do) by some tape and some self tappers into the door skin. You'll need to pull/fiddle to pop the skirts off the self-tappers. Mine came away easily (some weren't 'clicked' on anyway).
When refitting I am ignoring the self tappers (actually, I blobbed over them with some anti-rust paint). Make sure you've cleaned off all the old tape. Don't leave the plastic, cicular plug on the drive for the postman to stand on.
Check your sills for rust. Perhaps apply some Tetraseal or Waxoyl to the underside edge of the sills if they look ok.
Clean up the skirt. Refit it. Use something like Sikaflex 221 (buy it off fleabay). This stuff comes highly recommended. It should stick pretty quickly. You could lean something against the skirt as it gets a grip. Careful with this, of course.
That's about it. Not that difficult at all. However, if you aren't sure then give a few shops a call, and see what they quote you. Costs nowt to ask.
Let us know how you get on.