In the middle of the length of bias tube, tie a knot.
Then thread a polystyrene ball down the tube until it hits the knot.
Tie another knot keep the poly ball tight up against the first knot.
Repeat so you have 3 balls one side of the centre knot.
Then thread a poly ball and tie a knot on the other side of the centre knot. Repeat the process again so you have a central knot with 3 balls either side and 3 knots holding them in place.
Cut the ends of the tube off so they are level.
Tuck in the raw edges and slip stitch them closed. Give the ends a quick press to help them sit flat.
Now you can tie the ends together to wear your necklace.
It is very easy to use this technique to create longer necklaces but cutting and sewing a longer bias tube.