MLA 8th edition
Lastname, Firstname [or single username]. (handle). "First several words of Instagram post (if any)..." Instagram, Day month year posted, URL.
APA 6th edition
handle. (Year, month day posted). First several words of Instagram post (if any)... [Instagram post]. Retrieved from URL
Chicago 17th edition
Lastname, Firstname (@username). Year. "Full text of Instagram post." Instagram photo, Month day, year posted. URL.