。
My mother prepared dinner.
Literal
Mother [topic-は] dinner [possessive-の] preparations [object-を] did
用意をした = 'did preparations.' 用意 (ようい), 準備 (じゅんび), 支度/仕度 (したく) are near-synonyms for preparation, used somewhat interchangeably though with subtle differences in nuance — 用意 emphasizes having things ready, 準備 the orderly arranging, 支度 the personal grooming/getting-set sense.