Wash the cotton in the hottest water setting on your washing machine. The cotton fibers will absorb water and swell, causing the length of the material to shrink.
Dry the cotton on the highest heat setting available on your dryer. Quick and hot drying causes the shrinkage to become permanent. A cooler setting will cause the fibers to assume their original position.
Large amounts of cotton, such as cotton purchased by the yard, may need to dry longer and may need to have the process repeated again to ensure complete shrinkage. Make sure the material is hot to the touch and completely dry before removing it from the dryer.