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She picked out a scarf to go with the dress.
Literal
She [topic-は] that dress [matching-に合わせて] [to wear-つける] scarf [object-を] chose.
~に合わせて ('matching to / coordinating with') is built from 合わせる ('match, fit, coordinate'), with the て-form turning it into an adverbial phrase. Used in fashion, music, dance, scheduling — any time one thing is being aligned with another. つける here is a wear-verb specific to accessories that you 'put on' — scarves, badges, jewelry, glasses on a chain — distinct from 着る ('wear' clothes) and 履く ('wear' on feet). The whole 合わせてつける string acts as a relative-clause modifier of スカーフ.