To clean mustard stains, you first want to allow the mustard to completely dry. Once the mustard stain is dry, scrape off any excess mustard being careful to not scrape the stain into an unstained area. Once you have scrapped off as much of the mustard as you can, mix 3 parts of liquid dish soap with 1 part of a citrus-based cleaner. Pit this mixture on the stain and allow it to sit for 15 - 30 minutes. After, rinse the stain in the hottest water that is appropriate to the material being cleaned. The hotter the better, but don't ruin the material that is stained. Once you have rinsed the stain, the launder the material as you would usually and that should get rid of the mustard stain.