First of all, one can only take someone as far as they've gone themselves. Second, one must have an empathy for the beginner. Third, one has to know how to be available to answer questions without volunteering unrequested advice. Allow them to make mistakes!
It is easier for someone to dig themselves out of a hole they dug, rather than one the supervisor dug for them.
Mainly, internship supervisors should be available to answer questions, conduct code reviews, and help interns find tasks suited to them. I also recommend meeting with the intern periodically to ask them how they're doing.
The best support for my interns has come from former interns. In fact, I actually require anyone who wants to be involved in management to go through the same tasks as interns have to do.
I also formalized the internship with...
All that to say, usually the trick to finding internship supervisors is in making sure the expectations for the role are clear.
I hope that helps! (If not, feel free to follow up w/ additional questions.)
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