The cheapest, easiest and safest way is to use a tea or coffee dye! Brew up strong coffee or tea and soak your curtains to reach desired depth of color. (Remember, they'll look darker when wet, so maybe go a little darker than you'd like). To preserve the color, you can follow with a vinegar bath. If you don't plan to wash them often, you can skip this part, as the coffee/tea will essentially stain the material. I've used this technique for years with great & easy success. Good luck!