You will need:
Take a plastic bag, fold it flat making sure to fold the handle part flat too.
Take one end and fold it over and over until you have one strip.
Cut off the handle part making sure to leave a nice straight line.
Then cut the bottom seam off, this should leave you with a rectangle.
Open the bag and flatten again.
Now begin to fold over and over leaving about a 1/2 to 3/4 inch at the end (see pic).
You can measure and mark cut line to follow, I usually eyeball it.
Cut through the layera but DON'T cut straight through leave that 1/2" strip to hold it all together.
Now I find it easer to put my arm through the bag and have the loops on either side so that with my other hand I can cut the plarn.
You will cut on a diagonal, doing so will allow you to have one continuous strip on plarn instead of having to tie up each section. Makes it more "yarn" like.
Lastly you wind it into a ball and start your project! Have fun!