Fattening up guitars

Would anyone mind explaining how to set up a side chain compressor as mentioned? I would love to learn this trick.

well it depends what it is, but most sidechain compressors are mono output so with the one i have you set up a stereo group/fx track with what you want to be compressed sent to only the right channel and the key (what is making it duck) sent to the left side. then just twiddle with the settings as you normally would, the compressor should output you a mono signal panned center.