Rinse the cup with warm water immediately
Sprinkle baking soda into the cup
Add a little water to make a paste
Scrub the stain with a sponge or cloth
Rinse thoroughly with warm water
Use white vinegar for stubborn stains
Let the vinegar sit for a few minutes
Scrub again and rinse well
Use dish soap and hot water for light stains
Try a magic eraser on tough stains
Wash the cup in the dishwasher if safe
Repeat the process if needed