。
She put curtains on the window.
Literal
She [topic-は] window [on-に] curtains [object-を] attached.
つける ('attach, install, put on') is the everyday 'attach / install / fasten' verb, applicable to most physical attachments. For curtains specifically, つける and かける both work; かける emphasizes the hanging motion, つける emphasizes the act of installation. The window is the location/destination, marked with に.