1) Take any bag and fold it in half lengthwise 2 times, or more or less depending on the size of the bag.
2) Cut the bottom where the bag is sealed and the top handles off.
3) Cut the rest of the bag in strips of about 1″ or so. Wider if you want the plarn to be thicker and stronger, and narrower if strength of the plarn isn't really an issue.
4) Take 2 strips and loop them together in a knot as shown.
5) Pull the knot tight being careful not to rip the strip.
6) Continue looping the strips together making sure the knots are centered, (strips on each side of the knot should be even).
Another way of making plarn is to cut in at one end of the bag, and cut it in a long spiral strip all the way to the other end. This will result in fewer knots but its harder to keep the width of the plarn even. I prefer the above method.