It is simple. You can take white vinegar solution, one cup of lemon juice, and baking soda. Mix these ingredients into a small bucket. Put the curtains into the bucket, filled with solution, and leave it for at least thirty minutes. After 30 minutes, hand wash the curtains rub the stained areas of curtains to get the best result. But, this will help only to remove normal immediate stains and not the tough stains. For tough stains, you can hire professionals.