Teach your grandmother to suck eggs
Definition 1 of 1
Pro Tip 1/3Older Expertise
Use when someone older or more experienced is told something they already know. 
Explaining the tax system to Aunt Mary, a retired accountant, was like teaching your grandmother to suck eggs.
Pro Tip 2/3Potential Offense
It might offend if implying someone's advice is unnecessary. 
When I corrected his email etiquette, he snapped back, 'Don't teach your grandmother to suck eggs!'
Pro Tip 3/3Cultural Humor
This phrase can be humorous, especially when self-applied, mocking one's own overconfidence. 
I tried lecturing about classic cars until I realized I was teaching my grandmother to suck eggs.