Please put out your cigarette before entering the museum.
Literal
Museum [to-に] entering before [in-に] [topic-は] cigarette [object-を] extinguishing [polite-request-ください].
入る前には uses 前に for temporal 'before.' 煙草を消す means 'to put out a cigarette.' には adds contrastive emphasis — 'specifically before entering.' A polite rule.