Scrape off any excess mustard with a dull knife or spoon
Rinse the stain from the back with cold water
Apply liquid laundry detergent or dish soap directly to the stain
Gently rub the fabric together or use a soft brush
Let it sit for 5 to 10 minutes
Rinse thoroughly with cold water
If the stain remains, apply a stain remover or a paste of baking soda and water
Wash the garment in the warmest water safe for the fabric
Check the stain before drying
Repeat the treatment if needed
Air dry until the stain is completely gone
Avoid using heat until the stain is fully removed