Which phrase do you use to describe someone trying to do a thing that might/should be done better by you?
Solution 1:
This is known as teaching your Grandmother to suck eggs.
A phrase that indicates you are telling someone how to do something that they have done with great skill for years. The idiom dates back centuries, being used in Henry Fielding's Tom Jones, published in 1749.
Solution 2:
Probably not the exact answer, however I think it could be true in some similar situations:
Carry coals to Newcastle:
To carry Coals to Newcastle, that is to do what was done before; or to busy one's self in a needless employment.