Rinse the stained area with cold water
Mix baking soda with a little water to form a paste
Apply the paste to the yellow stain
Let it sit for 30 minutes
Gently rub the fabric together or use a soft brush
Wash the shirt with laundry detergent in the hottest water safe for the fabric
Add white vinegar to the wash if needed
Soak the shirt in a solution of oxygen bleach and water
Wash again after soaking
Dry the shirt in sunlight if possible
Repeat the treatment if the stain remains
Avoid using chlorine bleach on delicate fabrics unless the care label allows it