Start with the cord held betweem the first two fingers of each hand -
see diagram, then hook (towards you) the left side of the bight with
your right thumb, and then the right side with left thumb.
You should now be in the position illustrated in the diagram, now tuck
the two thumbs into the two triangles made by your first finger and
the cord.
Now here comes the tricky bit to explain: Twist your thumbs through
360 degrees, coming towards yourself at first.
This should leave you with a half hitch around each of your thumbs and
the cord. This bit requires a little bit of dexterity - pass the end of
each cord through the half hitch, and then ease the two thumb knots
together, and there you have a fisherman's knot.
Once you have mastered this, you should be able to get this down to
about five seconds without great difficulty. It is both a faster and
more reliable way (you can never get the thumb knots the wrong way
round) of tying the Fisherman's Knot.