You will need
Quite easyNever one to blow my own trumpet, you understand, it was a walk in the park for me but in the interests of covering myself, I'm going to say that it may be slightly more challenging for relatively inexperienced seamstresses.
TimeUnder an hour. Sorry to be vague. If I weren't photographing each step of the process it probably would have been minutes, but I'm not entirely sure.
Total cost£2 because I already had some elastic lying around and because b*tch, please, it's a scrunchie!
In the hair tonight...
Cut as long a rectangle as possible out of your scarf, measuring about 10cm in width (it's relatively easy to eyeball it). Turn it so that it's wrong-side-up (generally the paler, less glossy side), fold the ends back on themselves and iron them down.
Fold the fabric along the length - so that it's long and narrow - with the wrong side still facing outwards. Sew the edges together (not the folded ones).
slip-stitch the two ironed-back edges together.
Voilà! It's really that simple and quick. I just hope I haven't created a monster or helped to compound the felony if scrunchies really are back to haunt us from the atelier to the high street, at every conceivable level of taste! Just remember, if It's good enough for Marc Jacobs and Missoni, it's good enough for a crafter with a good eye and a scarf in need of recycling.