Emma Pygmalion is a bit disappointed with her results but she didn't get the method wrong. I use hair conditioner and sodium bicarbonate powder because I live in Adelaide and the water is very hard. Not all jumpers/ sweaters/ pullovers are going to stretch as well as you want. You can but try. Epsom salts is something else you can try and you can also add a tablespoon of lemon juice or vinegar. Know your local water.
Hair conditioner relaxes the fibres . I use about half a cup in a bucket of water. I use 2 tablespoons of bicarb. The water is 50C and I let the jumper soak for a good few hours. The conditioner and bicarb need to go to work. You will know if you have a chance . While the jumper is soaking , test it to see if you can stretch the wool. Don't pull hard. Pull gently. If the wool is stretching just leave it and try again. When you can get a nice , gentle pull it out of the bucket and squeeze excess water back into the bucket . You can do a second jumper now if you want to. Put into a plastic bowl to take outside.
Hang on a thick coat hanger and I take it outside in the shade. I start pulling the jumper where I want to stretch it. Pull long the seams before you stretch anything else. On a cardigan have it buttoned up. Pull to get the width and length you want but do it a bit at a time with a firm stretch.
Some jumpers will stay felted but fit better and it's not a bad look. Others will look less felted and then there will be the ones you need to turn into bags or mittens. I have more success than not. Patience.