After a ride the previous day, came out to find rear tire completely flat. Hadn't gone "whoosh" when I was riding, and didn't go "whoosh" when I pumped it back up, so it was relatively slow. If it goes "whoosh" and I can identify the location without taking the wheel off, I'll just take off a little section of tire and patch it without taking the whole wheel off, but since it was a slower leak, I had to take the wheel off.
After taking the wheel off and removing the tube, I inflated the tube and submerged it in a dishpan of water. Upon locating the stream of bubbles, I dried that section and marked it with a bullseye with a permanent marker. Pro tip: Marking in this way makes it easy to keep track of the location of smaller punctures after sanding the tube and applying glue.
I scuffed up the tube with a small piece of coarse sandpaper and applied a tiny dab of vulcanizing patch cement and schmeared it thin around the area. Upon allowing it to dry for 30 seconds until completely dry, I applied 1/4 of a Rema patch and pressed firmly for 30 seconds. Pro tip: Buy the box of 100 Rema patches, and cut them into quarters for patching the tiniest holes.
To avoid repeat punctures, I checked the inside of the tire for the offending sharp item. Pro tip: always align the logo on the tire with the valve hole in the rim to be able to know where on the tire to look for the pokey after locating the hole in the tube. I found a small piece of glass that had worked its way through, leaving a small cut about 1/8 of an inch long. The tire is fairly old, so is wearing thin and has more cuts like this, so will likely pick up more glass and get more punctures more frequently. I'm not sure if this will work, but I used some "Shoe Goo" adhesive to try to seal up some of the small cuts in the tire casing to prevent them from being weak points and picking up future glass and possibly slightly extend the tire life.
Reinstalled tube in tire, and tire in wheel, being mindful of tire logo orientation with respect to valve hole and ensuring that no portion of tube was caught between rim and tire. Upon reinstallation of wheel, rear cable-actuated disc brake was rubbing slightly indicating that the wheel was not sitting quite the same in the dropout as before, so I opened the quick-release back up and squeezed the brake lever while closing the quick release in order to make a tiny adjustment to wheel alignment in the dropout and ensure that the rotor was centered in the caliper.