If you have done some scoubidou you know the crown knot with four strands, the first picture below. You can also tie this knot with two strands at a time. If you do not know the knot, go to the website for more explanation. You can also tie this knot with more strands, one at the time, below there are pictures for six and eight strands.

If you tie six or more strands into a string it becomes a hollow tube. If you want the tube to be sturdy, tie the knots over more strands. Below the drawings for tying over three, but over two or more than three is also possible.

When you make bigger object you will find often the strands are not long enough. A good looking way to replace your strand is as follows. When one of your strands becomes short, place a strand in the center of the knot, (where the hollow of a tube would be,) tie a few more knots and then exchange place with the short end. Tie a few more knots and cut the left over bit short. With a bit of practice it is nearly invisible.

 
Have fun.