Don't you remember making these by the dozens and trading them as kids?
My girls have discovered them (through a little kit Kalla got for her birthday).
They are super easy to make, but a little tedious.
All ya need is some embroidery thread. To make the 4 color thick diagonal one you need 8 threads...2 of each color. If you want it wider, simply add more threads. I measured them all 25" long then knotted them together at the top.
You can use tape to secure them to a book, board or table, but I used a clipboard. Once secure for light tugging, separate all your colors in the order you want them.
Now, you start from left to right. Take each thread and double knot it to each of the other threads being sure to pull the knots all the way to the top. See that pink thread...I already tied 2 knots in the other pink, both greens and now am tying it to the blue. The pink goes around the back and then through the hole. Pull snugly to the very top. Keep going with each color til desired length. Tie both ends together. For my girls who like to take every off and on at whim, I tied it snugly around their hand then pushed it on. Now they can be pulled off and on.
Kalla did the skinnier bracelet much on her own. Still tough for a 7 yr old to keep all the threads straight. :) She still loves it (though coveting her sister's wider one) and wears it 24/7. Guess I can start now working on a few for the stockings (yep, already making plans for Christmas!)