It isnt really that hard. I guess a few tips would be,
First, if you havent taken on and off sidesills before, be careful, and up the front of side sills, there are 2 10mm nuts, you will have to slide your hand up inbetween the black vents in side sill, I use a open ended 10mm wrench.
Second problem people run into is they center ehaust tips in between side sill opening. But the exhaust grows a little bit when hot and so it moves back and burns back part of side sill. If you got the newest Corsa, the tip is changed a bit, its more round so it may not be as much of a problem. But ususally put the exhaust tip closer to front of opening,then when it grows,it wont touch the side sill.
Also, i like to get set up and clamped, put side sill back on and off each time to check but dont screw down. Once it is about where you think is right, then put side sill on nd tighten screws, recheck. If its fine, then I pull sill back off and spot weld a few sspots around clamp, that way it wont loosen up.
You can also use washers to space out the bracket that holds rear of exhaust by tip. This way you can get tips nice and flush with sidesill. Unfortunatley they take a little tweaking if your anal and want it to look perfect. Just take your time.
And have a sawzall on hand, its easy to just cut the stock exhaust crossover in to a couple pieces and then it comes right out.
Hope some of this helps. I could swear there was a installation guide with pics somewhere. Ill see if i can find it.