You will find below the explanation of the double fisherman's knot. Follow the steps 1, 2 & 3.
Pass the leather thread in the two holes from the two sides, so they will cross the running ends, putting the left one over the right one.
Pass the running end of the left rope over itself, then turn around the second running end and the first standing part from behind.
Pass the thread through the two top loops and tighten the knot. Cut the thread close to the knot. Do the same with the second free thread. And it's ready!