Having iTunes volume only affect Mac speakers, not those connected with Airport Express

In iTunes Help, it says "To use only the volume controls on your remote speakers, see Adjust the sound quality." Great! At Adjust the sound quality, it says "You can customize the sound for...specific speakers." Also great! But...

How do I do that? There doesn't seem to be any instructions on how to actually accomplish this thing iTunes Help is claiming I can do, namely to be able to change the main iTunes volume (through any of several methods) and have that not affect the speakers I have hooked up through my Airport Express.

I'm well aware I can do this by switching to iTunes, clicking on the Zoom button if I've got iTunes minimized, clicking on the little "speaker" icon (though how that looks like a speaker I'm not sure), choosing "multiple speakers," and dragging the "Computer" slider. OR, if I can figure out how to make iTunes do what the help linked above claims I can, I can just use my Synergy hotkeys to change the volume with a tap on my keyboard.