Ask HN: Finding idiomatic code or mentoring

Take a language like C++. How should an intermediate level programmer (someone who could code pretty much anything they wanted in the language) learn best practice?

One could recommend finding github repos, but most of them are terrible. Lots of people for C++ recommend, say, the Doom 3 code, but always add "but the style is c-with-classes; which isn't good modern C++ style"; one can just code it, but who knows what you'd be missing.

This isn't just about C++, of course, C++ is an example. One I've been thinking about because it is a language I've used constantly for 20 years, but I'm still not sure what people actually mean when they talk about 'good style'.

For those of you who feel like a 'master-craftsperson' in your language, how did you get there? Did you read code, or did you receive mentoring? Can you imagine a resource that would allow people to move up beyond intermediate level in a language? And how do you avoid the problem with self proclaimed 'experts' who love to teach more than they do learn, and fill the web with bad advice?