I love these necklaces in the summer, when its too hot to wear metal chains. You can make them any length you want and this technique would probably make a really nice anklet. About 2m of cord will make a short necklace, they do need a deceptively large about of cord. If it is too short you can add lengths of cord in, but its best not to as it can end up looking messy. Sorry I've got no pictures for the how-to, I hope it makes sense anyway.
Fold your cord in half. Make a granny knot where you have doubled the cord to make a loop. Test the loop to make sure it fits over your button, then tighten. You should now have 2 strings connected by a loop.
On the righthand cord, leave a short length, then make an overhand knot. Slide on your first bead. Make another overhand knot, so that the bead is sandwiched between 2 knots.
On the lefthand cord, make a knot at the end of the first bead. (This is so the beads down bump into each other and the necklace fits together better). Slide on another bead.
Knot the 2 cords together. Slide a big bead onto both cords, then knot underneith. (If your bead doesn't have a big enough hole for both cords, bring one round the bead and knot underneith).
Repeat steps 2-4 until your necklace is the desired length. I normally alternate the number of little beads that I put between the bigger beads, sometimes using 2 and sometimes using 3. If any beads have really big holes, thread the cord back through the bead to secure it.
When your necklace is the desired length, thread both cords through the button. Knot underneith the button with a double knot. Trim the ends and paint the knot with glue or clear nail varnish to secure. FINISHED!