To lower a car properly, is all I have to do buy and install a set of lowering springs?

I want to lower my Subaru wrx wagon preferably by myself without having to pay someone else. However I have heard that you can really mess up your suspension if you don't do it the right way. If I just install the lowering springs properly will I be ok or would I need to upgrade other parts as well? What are the steps I should take to properly lower my car without messing it up?