Mix together 4.5 litres lukewarm water, 1 cup ammonia, quarter cup baking soda, 1 cup vinegar.
Soak jumper in it. Pull into shape and leave in mixture 3-4 days.
Rinse and wash with vinegar and ammonia (the same as given before). Rinse again. Lay flat to dry.
This came from Fran Webb and is originally from HGA