put it in the trash... they stink. I own one and it is just a rack space filler. But, since that isn't what you wanted to hear.... You really have to do it by ear. "Why" do you want to compress....what are you looking to do to/for the sound? Attack determines how fast the compressor does the job of coming in once the signal reached the threshhold, and release tells it when to let go. Ratio is how much to compress for every db over the threshold.