Make one's bones
Look up make one's bones in Wiktionary, the free dictionary.
To "make one's bones" is an American English idiom meaning to take actions to establish achievement, status, or respect. It is an idiomatic equivalent of "establish[ing] one's bona fides".
Although the idiom appears to have originated in the United States criminal underworld, it has since migrated to more popular and less sinister usage; such as discussions of various professions and occupations including law enforcement personnel, the legal profession, and journalists.