teach your grandmother to suck eggs
US /tiːtʃ jʊər ˈɡrændˌmʌðər tə sʌk ɛɡz/
UK /tiːtʃ jʊər ˈɡrændˌmʌðər tə sʌk ɛɡz/

1.
ensinar a avó a chupar ovos, ensinar o padre-nosso ao vigário
to try to teach someone something they already know, especially someone who is more experienced than you
:
•
I tried to explain how to use the new software to my boss, but it was like teaching my grandmother to suck eggs; she's been using similar programs for decades.
Tentei explicar como usar o novo software para meu chefe, mas foi como ensinar minha avó a chupar ovos; ela usa programas semelhantes há décadas.
•
He started giving me advice on cooking, but I've been a chef for twenty years. It was definitely a case of teaching your grandmother to suck eggs.
Ele começou a me dar conselhos sobre culinária, mas sou chef há vinte anos. Foi definitivamente um caso de ensinar a avó a chupar ovos.