My cousin told me about this and I made some plarn. It was really easy. I started crocheting a rug with just eyeballing the widths of the plastic loops but then as I was working with it, I thought it would make a nice summer hat. And it does.
Basically you just neatly fold over 2-4 bags, with the ends to the left and top handles to the right. Measure and mark how wide you want the loops, then cut them. Take the loops and knot them together. You can make the loops any width depending on what you are making. Be careful to pull gently and hold near knot area when tightening knots to avoid tearing the bags. Right now I'm just using mostly white colored bags but the other day I went and organized my bags by color for possible future projects.
Here's a toddler's hat with my handwritten directions. However, I am still working on the accuracy with my instructions. As I might have mentioned before, I crochet while talking and keeping my husband company so I lose count a lot. Even though I try to mark down each group of stitches in a row. To make the sharp edges, you work in the back loops between the crown and side, then the side and brim. I believe the hat took about 14 bags. I'm working on a second toddler's hat which is smaller and should finish soon.