While it can be used as a presoak, it's actually designed to be used in the washing machine in place of laundry detergent. Add 1-2 ounces in the soap dispenser, warm water, normal wash, and extra rinse cycle (if equipped). No fabric softener or dryer sheets.
Oh, and using a couple ounces of APC in some warm water as a presoak is more economical and highly effective.