It's pretty easy - you'll be dealing with two spring bolts (or regular bolts if the factory spring bolts have been changed for whatever reason) and some rubber exhaust hangers. You might have to use some PB blaster on the spring bolts, a little lube on the exhaust hangers, and a fair amount of elbow grease on both, but it's really not a bad job if you have the correct size wrenches/sockets, a floor jack, and a set of jack stands (or access to a lift).
It shouldn't take you very long as long as everything cooperates. Make sure you use fresh gaskets, and pay particular attention to the type of gasket required between the downpipe and cat back (flat or factory/donut style).
What kind of cat back did you get or are you thinking about getting?
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)