Turn your sweater inside out. This protects the outside surface fibers from pulling.
Fill the sink with water (follow care instructions regarding temperature), add a small amount of a good quality detergent and mix it in slightly being careful to avoid creating foam suds.
DO NOT use chlorine bleach on wool because it breaks down the fiber.
Gently push the sweater down into water and then let it soak for 20 minutes; do not knead or agitate. Wool fibers are especially likely to permanently stretch if you over work the fibers; time is all you need to allow the water and detergent to penetrate and remove dirt and oils.
Rinse with tap water until no bubbles are left, being careful to hold and support the sweater from underneath as you lift it, rather than pulling it up, to prevent stretching. Repeat process.
Let the sweater drain for a few minutes and remove gently lay to lay flat on a towel.
Roll the towel, and then press the rolled towel, being careful to avoid wringing or rubbing the wet sweater. If excess water remains, carefully blot with a second towel being careful not to rub.
Lay face up on a dry towel and block or shape the sweater gently back into its original shape: pull together the waist and wrist ribbing, button cardigan buttons closed, roll collars into position, and straighten the sleeves.
Let dry overnight. In the morning, turn it over and block the other side; let dry.