a small piece of material attached to clothing or to something else made of cloth, with instructions on how to clean and care for it

Look at the care label first to see if the curtains are washable. 先看一下洗烫须知标签，看看窗帘是否可以水洗。

If it's a natural fabric you could dye it in the washing machine - follow the instructions on the care label. 如果是天然织物，用洗衣机洗时可能会染色——按照护理标签上的说明做吧。