Most plastic carrier bags are suitable. Sort bags into similar types (thick and glossy in one batch; thinner bags in another). Brightly coloured bags are great but any colours will do.
Trim off the base of the bag, then cut the bag across into strips about 1cm wide. Discrad the top portion of the bag, with the handle.
If you have followed the instructions so far, you will have a lot of large loops of plastic. Now you have to join them. To do this, slip one loop through another.
You are, in effect,linking loops to make a chain. Thread the loop back through itself and pull gently.
As you pull gently on both loops, the knot joining them will tighten. Now repeat the process, adding more loops to form a long chain. Then roll your plastic yarn into a ball, ready to use.