If you're like me and you have an old pair of tights you don't wear anymore, then congratulations, you have a thousand potential hairties (I have no idea how many hairties one pair of tights yields, please take a count and get back to me).
All you do is cut a strip off. That is literally it. I recommend cutting a thicker strip than you think you need, since it will fold in on itself and stretch when you use it.
Tights are stretchy so it makes a pretty big loop, but if I double it up before I use it, it fits perfectly around my hair. So go forth and chop up tights!