because I want to be better at explaining mathematical concepts (or any concepts, really) to others.

I'd suggest looking into effective techniques for tutoring, rather than teaching in general. It's both a more marketable skill, as well as more valuable for explaining things to other people. It may be my personality bias showing, though - I'm much more comfortable in 1 on 1 social situations.

As a strategy, I'd spend a few hours looking at how to evaluate the difference between good and bad tutoring, and then head up to anywhere that needs volunteer tutors and start volunteering.