How to tie a Fisherman's Knot
Take the end of each rope and place them parallel to each other. Take the end of one rope and place it over and around the second length.
As you come around place the end underneath itself, forming a overhand knot around the second length of rope.
Tighten the overhand knot.
Repeat the procedure at the other end using the second length of rope
Now the second length of rope has formed an overhand knot around the first length. Pull gently to tighten slightly.
Now pull on each rope so that the two overhand knots slide along the rope until they lie against each other.
If you want the final knot to be fully secure then tape the working ends of the rope down.