I take a big pot and fill it about 2/3 of the way with hot water and mix a big scoop of OxyClean in. You want to make sure that you leave enough room for the clothing in the pot. Remember they'll displace the water!
Bring your water to a boil and let it say boiling for 5-10 minutes. Use a laddle or spoon to stir your clothes and make sure that everything is covered.
Turn the water off and allow the clothes to sit until the water has cooled. Once the water has cooled set your washing machine and pour the clothes and OxyClean mixture in and wash as usual! If it doesn't come out on the first try a longer boil may be needed, try letting it sit over night and washing in the morning.