There may be another way to make it but this is how I figured out to do it.
You buy some of this stuff. It was only marked as 30c per mtr but I think it's nylon cording? It comes in sifferent sizes, decide on what size you want.
You set your machine on a straight stitch with your zipper foot on, get your material and fold about 1.5cms over your cording
Try to keep it tight as you sew
When you come to the end of your material, cut along the folded piece, DONT cut your cording, go back to the top of your material and fold it down about 1.5cms and repeat the same process.