Installing an Aftermarket Stereo?!

What are the steps I need to take? I have heard its very tricky, but when I look on Crutchfield.com, they show a direct fit Wiring adapter for the volvo. I want to know what Im going to get into before I install a stereo in my friends Volvo. I have installed Stereo's before, that is why Im doing research on this one before I ruin something! Thanks!

Pretty good site here for install/wiring.

Install (http://www.installdr.com/)