That's right, I'm talking about CDs (or DVDs). Those things your kids won't know how to use. If you think I'm dippy for suggesting that you turn valuable CDs into crafting tools, all I can say is what the hell else are you going to use them for? Your phone probably plays MP3s. Spotify and Netflix will let you stream basically everything ever. You're not "retro" by insisting upon owning physical media, and if that's your angle, go vinyl or go home.
Oh man, an LP would make a gigantic kumihimo disk!
Anyway, if you're still with me, go dig up a shitty mix CD an ex made you years ago, or if you're like my in-laws and you used to own a music store, just get one of your seven copies of Dido's No Angel. Time to cut that crap up.
Step 1: Materials
A kumihimo wheel template, printed from here. Set it for however many strands you will want to braid, and 12 cm diameter.
A CD or DVD. Probably not one of your Japanese Radiohead imports, but what about that old copy of Baldur's Gate in the basement? Alternately, get some crap media from the dollar store or thrift store.
A permanent marker.
A hacksaw or small handsaw.