- A knife or similar cutting tool. eg. A razor would be sufficient.
- A plastic soda bottle.
Cut off the bottom end of the bottle, and then make a small cut into the plastic at a 90 degree angle. Then cut from that point onwards creating a spiral strip of plastic until most of the bottle has been used, including using the funnel section of the bottle near the bottle cap. Cut the bottle cap section off when you have successfully turned most of the bottle into cordage.
If you need the cordage to be stronger, just make it a bit thicker.
If you need the cordage to be more flexible and are not worried so much about strength, cut it to be thinner.
DIY Plastic Cordage Cutting Tool
The video below shows how to make a cutting tool for making plastic cordage faster, although you really can accomplish the same thing with just a regular knife. Having such a tool is handy and faster, but unnecessary.