The single knot uses one corner of the furoshiki. You can gather the corner up neatly in one hand to make the single knot.
But if you are using an extra large furoshiki to make something like a shoulder bag, an attractive effect is achieved by folding the corner in on itself. Bring one side of the corner into the middle and repeat with the other side, once or twice, as if you're making a paper aeroplane. This creates a neat strap of fabric with which to create your furoshiki single knot.