I didn't take any photos along the way, but I think its pretty straight forward to explain, message me if you have questions.
I cut 6 layers of fine-mesh tulle, all rectangles about 4m long and 10" wide. You could use any fabric though - from jersey to taffeta - depending on how floppy or stiff you want it. If using something besides tulle, think about how many layers you want it, maybe just one if its a thick fabric.
Use a ribbon that's ~1/2" wide. For mine I used some leftover fabric and sewed it into a long tube.
I measured a doubled thread about 30" (secured at the end with a big fat knot, so it didn't go through the tulle) and used a long stitch (by hand) to sew the length of the tulle, right through the middle (diagram). As you sew down the rectangle, the tulle will ruffle.
Then lay it out so that the ruffles are distributed evenly. Mark the center of the ruffle (at the 15" mark) with a pin. Find the middle of the ribbon and pin it where you put the pin in the ruffle.
Continue to pin the ribbon to the ruffle, making sure the ruffle is even and the ribbon is straight & taut.
Sew the ribbon to the ruffle on the sewing machine. Try it on, make sure there's enough ribbon to tie a bow, and trim the excess ribbon, if any.
Wrap the ruffle behind your neck and then under your armpits and tie around your back.