Stereo Widening Using Channel Tool's M/S

Our sense of width in a mix depends entirely on audible differences between the left and right channels. Without them, we perceive a mono signal panned down the middle. Consequently, the best route to a wide mix involves careful attention to track and effect panning (and using primarily mono tracks). Even if you subsequently enhance the width artificially you'll always get the best results if the mix is already wide to begin with. 
 
There are two kinds of "stereo widening" techniques: artificially creating L/R differences via EQ, delays and phase shifts, and accentuating what differences are already there via M/S processing. There are plugins that can use more than one technique, including the Channel Tools plugin. There are also plugins that utilize M/S processing in conjunction with EQ (e.g. FabFilter Pro-Q, DMG Equilibrium) or compression (e.g. FabFilter Pro-C, DMG Compassion) to enhance width.
 
Basically what you're doing with the Channel Tools plugin (or any other M/S-enabled tool) is separating all the content that's different between left and right (the "S", or Side component) from all the content that's common to both left and right (the "M", or Mid component). If you then boost the former or attenuate the latter, the result is an emphasis on whatever components are different between left and right. It's important to remember that the plugin isn't adding anything, so if your mix doesn't already have some width to it, Channel Tools can't do much to help you.
 
Using Channel Tools is extremely simple: insert it into the fx bin of a stereo track or bus, leave the input mode set to "Stereo" and turn the "MID GAIN" knob down or the "SIDE GAIN" knob up. (I prefer to turn the mid down, to avoid raising levels.) That's it.
 
Channel Tools has some other tricks, too. The delay feature can be used to manufacture some L/R differences where there are none, by delaying one side relative to the other. Just watch out for very short delays, which can cause undesirable comb filtering. BTW, this is how the stereo enhancer in Ozone works, except it's more flexible due to having multiple bands.
 
Beware that often the side component contains much more high-frequency content than the mid, so boosting the sides may also make your mix brighter. In fact, a common cheat is to boost the treble in the sides in order to enhance width even further. Use with caution.
 
Personally, I almost never use Channel Tools or similar plugins for this purpose. If I do any post-mix widening, it'll be via EQ and/or compression (either M/S or simply unlinking a stereo compressor).