Cut two strands of yarn approximately twice the desired length. Lay them parallel to one another. Have another person hold one end of the piece for you. Roll the piece with your fingertips in one direction only, to just before the point where it starts to kink.
Holding the end of the piece with one hand, firmly pinch the center point of the piece with the other hand, and place the two ends together. Allow the piece to twist on itself. Gently straighten out any uneven kinking by working your hand along the twisted length. Knot both ends.
Another way to get an almost identical look is by simply working a length of chains; when you reach the desired length, fasten off and weave in the ends.