Fill a large basin with warm water. Add 2 tbsp. of baby shampoo. Mix the two ingredients together with one hand.
Submerge your wool jumper in the basin. Allow it to get sopping wet. Remove the garment from the basin by the top shoulder parts. Allow the water to run out of it naturally. The weight of the water pouring out of it will help weigh it down and stretch it naturally.
Lay a thick towel on a flat surface. Place the wool jumper on top of the towel. Press the wool jumper into the towel, allowing the water to seep into the terry cloth.
Pull on the ends of the jumper gently but firmly, coaxing it back to its original size. Go slowly, pulling on the seam of the wool, stretching it back to the appropriate size.
Press another towel on top of the jumper, soaking up the excess water and dampness once you've stretched it close to its original size or slightly larger. Allow the wool jumper to air dry.