What's The "Proper" Way To Set Up A Sub Output?

My audio device has 9 outputs, so I'd like to set up a subwoofer. Do I follow steps for a Surround Mix setup, and just leave out the other channels, or is there a more direct route to routing sub? Thanks!

Well there's pros and con's to sending to a sub from a separate output. One is if your end mix is headed for only stereo then you stand the chance of miss judging your low end in a mix. But if it is a multi channel mix then a dedicated sub feed is fine.
I only have 4 outputs and they all show in either the tracks or sub mix buss. So it's easy to send either a track or a buss if you go that route, but-  I use a mixer so my sub is fed from the AUX send. I have it on post fader so it is controlled by the channel strip I use for playback. I only turn it on when doing a low end check, otherwise wife and dog is disturbed and pictures need straightening.  

No, you can mix stereo with a sub. The whole idea is to supplement your main speakers by extending the lowest reproducible frequency downward. Otherwise, there may be stuff happening down there that you can't hear, stuff that might cause problems with your mix. If you can't hear it, you can't fix it.

The opposite is also true: if you hear too much of the very low end, that will certainly mess you up too. Your mixes will end up bass-light on normal playback systems. (The truism often heard is that whatever your speakers are good at is what your mixes will suck at.)

Many subwoofers also come with a footswitch that allows you to turn the sub off and on. That's a nice feature to look for when shopping for your sub. Your bass should sound good whether the sub is in or not.

Unless you're doing movie soundtracks, there's no need for surround sound. No one is making surround sound music albums...well, there's been a few, but it never really took off.