Opensim in the Classroom: How To Get Started

I've run sims with Opensimulator in my school for eight years now (!). I think it's immensely rewarding for students and well worth the effort. It's come a long way as a practical technology for schools. I'm not a techno-wizard but I'm so familiar with it now that it's pretty easy for me to run saved worlds or set up a sim from scratch, so I'm documenting the steps here for newcomers to the technology to use in their own forays into Opensim work.