、。
She was a tall, slender girl with long, soft brown hair.
Literal
She [topic-は] [tall-背の高い] [slender-ほっそりした] girl is-and, [long-soft-brown-長いやわらかな茶色の] hair [object-を] was-doing.
Two notable patterns. 背の高い uses の as a substitute for が in noun-modifier position — common in relative clauses ('whose height is high'). 髪をしている is the standard descriptive 'has (a feature)' — 髪をしている = 'has hair (of a certain kind),' 目をしている = 'has eyes (of a certain kind).' This idiomatic ~をする pattern is how Japanese describes innate features.