It's not hard to ruin a knit sweater. Whether you wash it wrong or dry it wrong, you can easily shrink it, stretch it or cause it to unravel. But since dry cleaning gets to be expensive after a while, many people wash their knits by hand. Washing them safely isn't hard, but drying them out is a bit of an art form.
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If you've just washed your sweater, it should be soaking and dripping. Many people's inclination is to wring the water out, but you should never do this. Wringing out wet knits almost always causes the knit to stretch and take on an odd shape. Instead, you should gently squeeze some excess water out of the sweater without wringing or twisting. Just bunch it together a few times and spend a little bit more time squeezing than you would wringing, since squeezing isn't as effective.
When most of the excess water has been squeezed out, lay a bath towel flat and open on a table or other flat surface. Spread the sweater out flat on top of the towel.
Carefully roll the towel up with the sweater inside. Make the roll snug, but not overly tight. This process will squeeze even more moisture out of the sweater, and it will be absorbed by the towel.
Unroll the towel and take the sweater out. It should just be damp at this point. Lay the other towel out flat and spread the sweater out on top. It just needs to air dry the rest of the way.
After a couple of hours, turn the sweater over so that the other side can air dry.
To speed up the air drying at the end, you can focus an electric fan on the sweater. This will usually get knit sweaters completely dry after about one hour of air drying per side.
Make sure you use colorfast towels, and preferably ones that have been washed several times before. Brand new towels may transfer dyes to your sweater.