Wow! Even the title of this post started to make my brain seize up before I was even done writing it. The phrase "bored already" comes to mind.
But I left it that way because folks who have done lots of recording before will understand what a submix bus (sometimes - even by me, depending in part on how much caffeine I've had - spelled "buss") is.
If you don't know what it is, see my article What is a Buss in Audio Recording?
OK, now I expect everyone to know what a submix bus is. You DID read your assignment didn't you? I thought so. In most multitrack audio software programs (usually called digital audio workstations, or DAWs for short), if you want a submix (like the example of "one track to rule all the harmony tracks," remember? From your reading?), you first have to create a special kind of track - a bus.
Then for each of the tracks whose audio you want inside the submix, you have to disable the "sends" going to the master bus. That's because a bus track merely taps the audio on its source tracks - like creating a clone of it. It doesn't stop those tracks from ALSO sending their audio to the master track.
If your goal (as it often is with a submix) is to be able to control the volume of ALL the tracks in the submix group (like all the harmonies, say) with just a single knob/slider (the one on the submix bus track), then ALL of the audio from each of the original tracks needs to be routed through the submix track. Otherwise (I'm going back to the clone simile here), you could make the harmony clones be quiet by turning down the volume on the harmony submix track.
But the originals - the "not clones" - are still yapping away, making all kinds of loud harmony-type sounds. Your submix track can't control the originals! So you must destroy the originals (Insert dramatic music here) by disabling those "sends" headed for the master bus.
In Reaper - my favorite DAW - they have something called track Folders. They make it soooooo (is that enough "Os"?) easy to create a submix. Here is how you do it. Don't look away because you might miss it (it's THAT quick and easy)!
It took longer to type that than to actually do it. Assuming you have the correct mix (relative volumes) of the harmony parts against each other, all you have to do to control EVERY harmony in the folder track - put on reverb, turn them up or down in volume, compress them, etc. - is do it to that top track.
As I mentioned above, the tracks that get put into a folder are shown as indented (see the picture on the right), and are considered "child" tracks in the folder. The folder track at the top is the controlling or "parent" track for the submix created by the folder.
Using folders in Reaper is a HUGE time saver, and the best way to create a submix that I know of.
By the way, you can still do everything in Reaper "old school," like creating "sends" from each harmony track and directing them to a new track that you call a "bus," etc. if you want to.
Maybe you have some special need for doing things the hard way, or maybe you want to teach someone how it used to be done in the old days. Some folks may want to do it this way for a very specific and unusual effect or something. But as far as I'm concerned, folders are the way to go!