teach one's grandmother how to suck eggs
tell someone how to suck eggs
try to tell or show someone how to do something they can already do well
A person who is fond of telling others how to suck eggs is never well received.
Standing in front of you and telling you how to suck eggs is really embarrassing.
I don't want to tell you how to suck eggs, but you could try using this one.
The expression 班门弄斧 literally means the ordinary craftsman is showing off his skills with an axe in front of Lu Ban, the master carpenter.
The phrase teach one's grandmother how to suck eggs is a reprimand to someone who, although inexperienced, thinks it necessary to lecture a wiser person. It alludes to the fact that a toothless grandmother will be naturally more successful in sucking the contents from an egg than a grandchild with a complete set of teeth.