Let them setup an environment s.t. they can work on the codebase - help them, but also let them document it for the next person (or update existing documentation).

Give them a high level overview of architecture and best practices.

Let them work on very small features/bugs (update text, fix small issues, ...) s.t. they learn the process end-to-end without spending much time on code.

[^ everything until here could be done without human explanation if you have solid documentation and codelabs]

Give them small features to work on s.t. they can own part of the codebase.