You need prefit it about 300 times while you drill the holes and fix any cracks in it and possibly reinforce key area with extra glass for extra protection. And most aftermarket front bumpers don't have supports holding the bottom of the bumper to underneath the car like stock ones do. So you need to get something like this:
To hold them from underneath to chasis or all the weight will be placed on the sides that hook up the fenders and eventually snap them apart.
As for the side skirts, depends. Some ppl have custom brackets. I myself used various bolts to hold it from underneath to existing holes I found in the chasis. Just gotta get in there and look around for all the support it needs.