The best way to get mustard stains out of clothing is to take some Liquid Laundry Detergent such as Tide or even Persil or other heavy laundry detergent and pour some of the laundry detergent onto the mustard stain.
Then rub the laundry detergent onto the mustard stain and into the mustard stain and allow the laundry detergent to sit on the mustard stain for up to 10 minutes.
After 10 minutes have passed you can use some water to rinse the mustard stain away and rinse the laundry detergent away.
Then blot the stain with some vinegar or water to help lift the mustard stain completely out of the clothes.
After that is done then put the clothes into the washing machine and wash the clothes to remove the mustard stain and mustard and laundry detergent completely from the clothing.
If the mustard stain is new then using some Tide To Go on the mustard stain works well to remove the mustard stain and other food stains.
But the longer the mustard stain sits or the longer the other food stain sits the harder it is to go out and then Tide To Go does not work as good.
You can also sprinkle some baking soda onto the mustard stain and then pour a little vinegar onto the mustard stain and baking soda and allow it to sit for 5 to 10 minutes.
Then use a toothbrush to scrub the mustard stain out of the clothes and blot it with a washcloth and then wash it as normal.
Soaking the clothes in Oxiclean also works to remove mustard stains whether dried mustard stains or fresh mustard stains.