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As a general rule, you should use 3 gallons (12 L) of water for every 1 lb (450 g) of fabric.  The water should be simmering hot when you pour it into the tub.  Glass and stainless steel will not be stained by the dye, but most plastics will become stained. If you are worried about staining the tub, consider lining it with plastic sheets before filling it with water. This works best if you are using a single tub. If you need to split the process up between two tubs, though, make sure that the amount of water and the amount of dye you add to each tub is exactly equal. Alternatively, you could use your washing machine to dye the curtains. To do so, begin by filling the washing machine basin with the hottest water possible. The rest of the process will essentially follow the same steps. There are differences between liquid and powder dyes, and there may even be further differences between dye brands. Check the instructions on your dye to determine the best way to prepare it.  Usually, you will need to prepare a bottle of liquid dye by vigorously shaking it for a minute or so. To prepare powder dye, fully dissolve one packet in 2 cups (500 ml) of very hot water. Dump your prepared dye into the filled basin or filled washing machine (whichever you decided to use). Use a paint stick or board to stir the dye in until you are certain that it has fully spread throughout the water. If the curtains are dry or cool to the touch, quickly soak them in a sink or separate tub filled with clean hot water. Hot water helps activate the dye. Your results will be as pure and even as possible if both the dye bath and the curtains are hot when you transfer the material to the dye. Place the curtains in the dye bath, submerging them completely beneath the water's surface. Allow them to sit in the hot dye for 5 minutes. Do not stir or agitate the curtains during this time. If you are using a washing machine, do not start any sort of washing cycle yet. After the first 5 minutes pass, add 1 cup (250 ml) of salt or white vinegar to the dye bath for every 3 gallons (12 L) of water. You should also add 1 Tbsp (15 ml) of liquid washing detergent.  Salt and vinegar help intensify the dye color. Use salt with cotton, linen, ramie, and rayon. Use vinegar with silk, wool, and nylon. Liquid detergent allows the dye to move more freely throughout the water and into the fabric fibers. Once the additives are in the water, allow the curtains to soak in the dye bath for about two hours.  This amount of time is standard if you want to produce the intended shade; however, you can leave the curtains in for a shorter or longer time if you want a lighter or darker shade, respectively. Check the curtains periodically until you achieve the shade you want. Note, though, that the final shade will usually be a bit lighter than it appears when wet. Agitate the curtains continuously. If you are machine dyeing the curtains, set the machine on an agitate cycle and continue agitating the fabric the entire time. If you are dyeing the curtains in a tub, stir the fabric every few minutes using a large painting stick or board.

Summary:
Fill a large tub with hot water. Prepare the dye. Mix in the dye. Soak the curtains. Place the curtains in the dye bath. Add salt or vinegar. Soak for several hours.