A needle knot is used to attach the leader to the fly line.
The tools required are as follows.
a pin vise
a sewing machine needle
a spring-type wooden clothes pin
a file
a round tapered tooth pick
a finger nail clipper
The process is as follows.
Insert the needle in the pin vise and tighten.
Take apart the clothes pin and put it back together the
wrong way, use the file to flatten the wood where the
clothes pin meets.
Place the end of the fly line in the jaws of the clothes
pin.
Push the needle through the center of the fly line about
3/8 to 1/2 inch. You may need to place some pressure on
the clothes pin to keep the fly line from slipping.
Remove the fly line from the clothes pin and force the
needle out the side of the fly line.
Thread the small end of the tapered leader through the
eye of the needle and pull the needle and the leader out
of the fly line.
Pull the remainder of the leader out, leaving about 10
inches.
Tie a nail knot about a half inch from where the leader
exited the side of the fly line. Make sure that the fly
line has not been damaged where you tie the nail knot.
To tie the nail knot, grab the fly line and the leader
about 3/4 inch from where the leader exits the side of
the fly line. Place the tooth pick along the fly line and
leader. Allow a little less than half of the tooth pick
to protrude.
Wind the tag end of the leader around the fly line, the
tooth pick and the leader. Make 5 or 6 tight wraps. The
secret of tying a good nail knot is to make sure the
wraps are tight. Don't allow the wraps to unravel even a
little bit.
While holding the wraps pull the tooth pick out a little
so that the wraps are still around the tooth pick.
Poke the tag end of the leader through the loops while
holding the wraps firmly.
Tug both ends of the leader to tighten the loops, still
holding the wraps. You may have to do this a couple of
times to pull the slack out of the leader.
Pull the tooth pick out completely and again tug on
either end of the leader. When it is snug you can let go
of the wraps.
Moisten the knot and pull hard on either end of the
leader. Trim the tag end of the leader.
For extra strength place a drop of super glue on the nail
knot although this is usually not necessary.