I'm thinking about this a fair bit at the moment, because I'm trying very hard to make myself obviously approachable as a mentor for DW volunteering. Some of the things I am doing include:
- keeping an up-to-date list of bugs for beginners in a prominent place
- making sure that every time I say anything in an I'd-like-to-mentor-you capacity I give a list of ways to get in touch with me, and say "please feel encouraged" (rather than "please feel free")
- talking in public about mistakes I've made or things I've found overwhelming and about how I think volunteer environments of this kind ought to feel

I don't know what else it should look like; this is kind of new to me; but I am trying, and it does seem to be working, both in terms of getting people I know who've been scared to try involved, and I've had several complete strangers rock up and say "on the strength of that thing you wrote, I want to give this a go", or "this thing you are doing is useful, thanks, but I don't need anything from you at the moment!" -- which -- is all really, really lovely.