Dinnae teach yer granny tae suck eggs
(Don't teach your grandmother to suck eggs) •
Don't try to teach someone something that they already know; mansplaining. Your granny already knows how to suck eggs, there's no point in trying to teach her.
"You make tattie scones by..." "Pal, dinnae teach yer granny tae suck eggs! Av been makin em fair yonks?"
"You make potato scones by..." "Mate, don't teach your granny to suck eggs! I've been making them for years!"