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Check the tag and use the hottest water safe for the item of clothing you're treating. If you're supposed to wash the garment using cool water only, use cold water instead so you won't warp your clothing. Concentrated liquid detergent is powerful stuff, and it should take the stain right out. Use it only for fabric softener stains that are especially large or stubborn. Leave the garment for a few minutes to let the detergent soak into the stain as a type of pretreatment. Use hot water if possible, but if the garment says "cold water only," you'll have to go with that to prevent damage from occurring. Add the same detergent into the washing machine that you used for pretreatment of the stain. Once the drying cycle is finished, the stain should be completely gone. If you still see a fabric softener stain, repeat the process.
Wet the stain on your clothing with warm or hot water. Rub liquid detergent into the stain. Let it soak. Wash the garment in the hottest water that is safe. Dry the garment as normal.