After cutting off the top and the bottom, lie bag flat in front of you with the open edges at the sides (after cutting off the top and the bottom it is like a tube). Roll up from one of the ends that is not cut like in the picture below.
After this stage there is a way of cutting the bag , kinda like the way you peel an orange in one go.
I got the inspiration for this technique from a book I borrowed from the library, the projects in it are a little tacky – which is easily done when knitting with plastic. But there must be a way to knit things with plastic in a non tacky way!
I also tried recently to fuse plastic together but I do not have the right type of baking paper ( greaseproof paper does not work very well and sticks to the plastic)