How do I turn a sweater into felted material?
I haven't tried this myself but from what I understand it's a pretty straighforward process. First off make sure your sweater isn't an acrylic blend - it might not felt. So 100% wool is best. Then you can put it in the washing machine with a bit of detergent - put it on a hot/cold cycle or a warm/cold cycle. The hotter the water the more it should shrink. If you are looking for a certain amount of shrinkage you should check the progress as it washes. Then you can air dry it or put in in the dryer on low. A hot dryer might cause it to shrink more. As the fibers shrink they will become more dense too.
If it doesn't felt as much as you like you can try putting it through another wash cycle. Some tips I found on the net are: do a pre-soak to open the fibers, put jeans in with the sweaters to agitate the fibers more, softer wools will make lighter, more drapey, material. Finally - I read that some wools just won't felt - if that's the case - and you bought the sweater from the thrift store just donate them back!
Hope this helps!