If you wanted to teach someone to sing, how would you go about it?
No, I'm not entirely sure what I mean by "teach someone to sing", which is part of the problem... IME most people can sing (and when they say "I can't sing" what they usually mean is "someone told me when I was a child that I couldn't sing"); what they can't necessarily do is stay in tune (with others, or even with themselves). So let's say you want to get somebody to the point where they're able to do that well enough that they can join in confidently with 'community singing' (weddings, carol services, etc.), and eventually do simple part-singing. Where do you start? Am I asking the wrong questions?
Reading music is sort of orthogonal (and the sort of people I'm thinking of could probably teach themselves that fairly easily anyway, because they're bookish kind of people).