How would you write a guideline for junior developer

1. Documentation goes a long way
2. Even further is a dedicated mentor
3. For things that can be automated (style checks, common programming patterns etc.) they should be automated for consistency
4. Let them work on projects so that they can apply their knowledge, let them experience it

My take on your question

Any documentation is better than no documentation. Get in the habit of writing development notes a la Wiki and make it accessible to the whole team. My biggest issue when joining as a junior dev was that I had to ask even the simplest questions about the codebase because there were no notes or documents on the application at all.

I've found its very helpful to have a set of tools and documentation you can have them try to conform to right away, with the guidance of a dedicated mentor.