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She designed beautiful costumes for the movie.
Literal
She [topic-は] that movie's [for the sake of-ために] beautiful costume [object-を] design-did.
~のために ('for the sake of, for') here marks the beneficiary or purpose of the design work — for whose use the costumes were made. のために (with the noun-modifying の) is distinct from the bare ために (verb-modifying), which means 'in order to'. デザインする is one of countless loanword + する verbs Japanese coined by sticking する onto a foreign word: ファクスする, クリックする, シェアする — productive enough that virtually any English noun can be verbed this way. 衣装 specifically means a costume, stage outfit, or formal attire — distinct from everyday 服 ('clothes').