Schuster, bleib bei deinem Leisten!
Shoemaker, stay with your last. The dictionary DWDS explains: do only what you understand something about.
Das Ei will klüger sein als die Henne.
The egg wants to be cleverer than the hen. It is said of a young person who wants to teach older and more experienced people.
Sind Sie vom Fach?
Are you from the field? The question asks whether someone is a trained specialist.
Er versteht sein Handwerk.
He understands his craft. It is praise for anyone who has mastered the work, not only for craftsmen.
der Quereinsteiger
The lateral entrant. Someone who enters a profession without its usual training.