Springs are easy sleazy... helps to have an extra set of hands and warm, well lit area to work in. How much will the WRX springs lower your impreza? You may need to trim the bump stops a wee bit.
Look on Scoobymods in the Forester suspension mods for a step by step procedure. Another KEY thing to remember is the order of parts that go on and the direction that the strut top faces. If I had to do it again, I'd used that grease pen or a sharpie to draw a line all the way down top to bottom of teh strut assembly to mark alignment.
Also, before you remove the strut, mark one of the strut top bolts and put a corresponding mark on body of the car so you know how the top hat realigns.