。
She decorated her room with roses.
Literal
She [topic-は] her own room [object-を] roses [with-で] decorated.
バラ ('rose') is conventionally written in katakana — Japanese flora and fauna names often appear in katakana when they cross over from technical or scientific writing into general use, even for native species. The instrumental で marks the roses as the means/material of the decoration; 飾る ('to decorate') takes the place adorned as direct object (を) and the decorative material via で.