Remove the collars stays from the dress shirt before you do any cleaning or laundering to prevent them from getting lost or falling out in the machine.
Soak the dirty shirt in a tub of cold water.
Spray the stain on the collar with a stain-removal pre-wash spray, or rub it with a stain removal stick. Always follow the directions on the particular product that you use. Rub some shampoo into the stain if you don't have a pre-made stain removal product.
Rub the stain gently with a toothbrush until the collar appears clean.
Wash the shirt according to the wash instructions on the label.
Examine the collar carefully. Make sure that the stain is gone before you dry it, if it is a shirt that is machine dried. Heat in a dryer causes some stains to set, becoming permanent.