I do it with thrift store sweaters and make mittens out of them. You can use a top loader or a front, it doesn't matter. What matters is that you use pure wool (though I've had success with sweaters up to 80% wool) wash in hottest hot, WITH detergent, rinse in coldest cold and dry on hottest hot. I have had to do a few three times. Part of the felting comes from the agitation of the machine.
WARNING - I've made lots and lots of mittens and this plays havoc with your sewing machine. It will fill up with lint.
Here's a link to the website I used for my mitten pattern:
I've manged to get 5 or 6 pairs of mittens out of a man's size large sweater by adjusting the pattern to make men's, ladies and several smaller children's sizes. The hardest part is deliberately throwing some of those beautiful sweaters in the wash.