How to know when ready to NG? Depends a lot on goals -- some projects need ops; others need docs/QA. Need to do something in C, C++, or Rust with mentorship from someone experienced before jumping into NG. It also requires the meta-skills: Human interaction, installing a toolchain, reading/leraning from docs are the hard technical prerequisites.
NG cannot teach intro-level culture -- issue queue with good habits, Git with good habits (clone/push/pull; expect to teach rest of revision control), IRC manners, dev mailing list manners. Can teach USENET but they must learn fast and behave properly. Cannot teach WHEN TO ASK QUESTION -- raw newbies often get this wrong. "how do I participate in a project like an adult" is the prerequisite, plus not getting feelings hurt by direct code review and understanding need for failure.
If Susan can say "read this tutorial, do it, try the task, tell me what else you need to know for the task" and you can self-manage, you're probably good. Plus adminning own machine and maintaining toolchains. And compulsive documentation.