There's many different ways of using compression, but I'm going to focus on its traditional purpose, to help tame the dynamic range of your song, or more specifically, a part of the song. As with all my guides, I think it's important people realize I'm not saying "this is how it's supposed to be" or that there's no other way to use a compressor. Certainly they've been misused to great effect and fame. Think of the Benny Benassi basslines, or that Ministry would often use racks of compressors to achieve distortion for their guitars, and not dedicated distortion units. What I'm saying is, don't take my word for it, and don't just read this guide and think you have a handle on how to use compression. It's one of those things that's often very subtle to hear if done correctly, and it's something that takes a lot of hands on practice with to get a real understanding of. In this guide, I'll mainly be showing you how I learned to use compression, there's certainly many other ways to approach it though. That said, let's dive in.
Article: Dynamics Control for beginners