Make a paste of 4 tablespoons of Baking Soda to about 1/3 cup warm water, use less if to watery, use more if not watery enough. (I just eyeball things sorry)
Rub the paste on the ring-around the collar area. Let it sit while sorting your laundry into piles, just before you start the load with the shirt that has ring-around the collar, pour or spray a little white vinegar over the paste throw into the washer and start the load.