"Essential Knots: The Double Fisherman's" is part of the book - Trad Climbing Basics.
Loop one end of the cord around the other end twice, then push the end through these loops.
Pull it tight. Make sure the tails are at least ten times the diameter of the cord (5cm tails for 5mm cord).
Do the same with the other end of the cord.
Pull it all tight so that the two knots jam together. It is important that the double fisherman’s knot is fastened tightly.
Triple Fisherman's Knot
Some cord or rope may be too slick or stiff for the double fisherman's to work. In this case, try adding an extra loop to each side of the cord to make a triple fisherman's knot.