Step 1: Felt your sweaters
To felt the sweaters toss them in your washing machine on hot with detergent and run an extra cycle. Dry in the dryer. Repeat if necessary. The sweaters should be well shrunk and thick--they should not fray when you cut them.
In addition to the sweaters, you'll also need sharp scissors and a sewing machine. Ready to go?
Step 2: Cut out your circles
I can't say exactly how many you need but I spent about 2 hours cutting up most of 3 sweaters and ended up with 13 feet of garland. Maybe cut until your hand is tired and see where you're at?
Note the ruler for scale. You want a mix of sizes and shapes to add interest.