As in the title, I'm looking for an expression/idiom where a novice tries to lecture an expert on their field of expertise.
To try to teach an older person who is wiser and more experienced and worldly than a young person may think. Why are you explaining basic typing to Ethel? Yes, she's 70, but she's been using a computer since before you were born—quit teaching your grandmother to suck eggs.
Or, if you prefer:
Teach fish how to swim is an idiomatic expression derived from the Latin proverb piscem natare doces. The phrase describes the self-sufficiency of those who know better how to do everything than the experts. It corresponds to the expression, "teaching grandmother to suck eggs".
I've forgotten more than you'll ever know
but I am unable to find a reputable reference for it at the moment.