Complex sentence: Young Sidney got a job as a dressmaker, and spent her spare time studying law and writing fashion articles, leading to a job as a "mannequin" (or model) in a Fifth Avenue shop.
Split into simpler sentences: Young Sidney got a job as a dressmaker, and spent her spare time studying law. She spent her spare time writing fashion articles, which led to a job as a "mannequin" (or model) in a Fifth Avenue shop.