Pour a pea-sized drop of liquid detergent on any stains on the collar. Rub the detergent into the stain.
Place the item in the washing machine.
Check your oxygen bleach package to ensure it is safe to use on all fabrics. Pour oxygen bleach into the washing machine, according to manufacturer's instructions.
Add laundry detergent according to instructions on the package.
Turn on the washing machine. Use the hottest setting tolerable for the dark-colored shirt. Fill the machine with plenty of water. Too little water will cause stains and particles to resettle in fabric, rather than washing away.
Check the garment for residual stains. Retreat the stain and rewash until the item is clean.