I've started a new custom order...making a bag from two vintage sweaters. First step of the process was to mend a number of holes in the sweaters. This is important because the holes can get much larger when the sweater is felted. If the holes are small, the mending won't even show in the finished felted sweater., but these sweaters had some pretty large holes.
The next step is to felt the sweaters. I do this in my top loading washing machine. Set the water on the lowest hottest setting. Place the sweaters in zippered pillowcases (to save your washer). Throw in a small amount (1/8 cup or so) detergent, some tennis balls, and/or and old pair of jeans for agitation. Check the results every 5 minutes or so. The sweater is "done" when you can no longer see the individual knitted stitches. Allow washer to spin briefly to remove most of the water, remove sweaters and hang or lay flat to dry, pulling them into shape as needed.