Preparing plastic bags is pretty simple.
Start with a plastic bag.
Fold it in half.
Fold it in half several more times.
Cut the handles off.
Cut the bottom off.
Cut the bag into about one-inch strips.
You should end up with about 10 pieces per bag.
Unfold 2 pieces and loop them together.
Loop all the bag pieces together to form a chain.
Roll the plastic bag chain into a ball.
I typically wait until I have a lot of bags. Then I cut them all up and end up with a crazy mess like this. Then I roll this mess into several large balls.