For the spiral cutting method of making plastic yarn, as before, you'll trim off the top and bottom of the bag. Working with the bag in a single layer rather than folded, pick a point (such as along one of the side seams) to cut down into the bag to the depth that you want the width of your yarn to be, then begin cutting around the bag, cutting and spiraling down as you go.
Pay attention as you work your way around the bag, and if it ever looks like you're about the cut the strip so it will no longer be attached to the bag, just cut at an angle to give yourself more room to work.
Of course it's not the end of the world if you do cut the yarn off from the bag before you've reached the end; you'll just have a couple of shorter lengths to work with rather than one piece as long as the whole bag.
While cutting the bag in this manner, you might find it easier to work with the bag over your knee so that you can be assured you're only cutting through one layer at a time. I found this to be faster than cutting while holding the bag in my hand or on a table.
Whichever method of plarn making you choose, wind your strand of plastic yarn into a ball when you're done and you'll be ready to knit.