。
She waved goodbye.
Literal
She [topic-は] hand [object-を] waving said-goodbye.
さよならする (to say goodbye) treats the farewell as an action done with する. 手を振って connects the physical gesture with the farewell — waving as the manner of saying goodbye. さよなら written in hiragana is the standard form; the kanji 左様なら (if that is so) reveals the word's etymological origin as a formal parting acknowledgment.