Place the cotton item in a washing machine and add the requisite amount of detergent. Close the lid and select a hot water wash and rinse for the entire cycle. The detergent will remove all the coatings and together with the hot water cause the item to shrink.
Remove the item from the washing machine and place it in a dryer. Set the dryer to the highest heat possible. Allow the item to have a full dryer cycle of at least 60 minutes. Drying it over the highest heat will cause the item to shrink quickly.
Take the item out of the dryer and fold it, placing it in a shelf of the closet or drawer. Do not hang the item up in the closet as the suspension from the hanger can sometimes cause it to stretch slightly.