Lay the no-iron shirt on a flat work surface covered with a clean towel. Position the collar so it's open.
Wet the collar soil stain with lemon juice. Allow the lemon juice to soak into your shirt for 20 minutes to loosen the oil and sweat stains.
Rinse the collar with warm water and wring it out. Place 1 tablespoon of powdered laundry detergent in a bowl. Add enough water to make a smooth paste.
Spoon the detergent paste onto the remaining collar soil stain. Work the paste into the fabric by scrubbing it lightly with the toothbrush. Allow the paste to sit on the fabric for 10 minutes.
Rinse the paste off the shirt collar. Wring out the fabric. Pour white vinegar onto a sponge and dab at the remaining collar soil stain until removed.
Launder your shirt in warm water. Add laundry detergent and all-fabric bleach to the wash cycle to remove any remaining ring around the collar. Hang your shirt to dry.