Tom, for a start, take a piece of string, say, three feet long. Make a loop, say, four inches in diameter, near the center. Put the right end of the string through the loop from top to bottom and pull it all the way through. Repeat, repeat, repeat, so your string coils around itself in a helical fashion. Keep going until you have used up the whole right end of the string, leaving an inch or two free. Then do the same thing with the other end of the string. When the ends line up for the last time, tie them in a knot, cut off the excess, and poke the cut ends down into the middle somewhere.

If you want to get fancy, you'll have to figure it out for yourself. It's only as complicated as you make it.