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".