Lay out plastic bag and smooth out the wrinkles.
Cut off bottom seam.
Cut bag into 1 inch strips.
Your strips should form rings. Now take 2 rings...
And lay the left ring partly on top of the right ring.
Tuck that part of the left ring under right ring...
And pull up, tightening the knot.
This is called a Loop to Loop Knot.
Keep looping rings to each other to form a chain.
Use this plaastic chain as your "yarn".