How to tie Nail Knot?

Nail Knot use to attach leader butt to fly line or to attach backing to fly line

Steps to tie Nail Knot,
1- Take a hollow plastic tube like a piece of drinking straw, and lay it next to the
    end of fly line. Then lay the butt end of leader on top of tube in the opposite
    direction of leader butt extending beyond end of the tube.
2- Hold the fly line, tube, and the tag end of leader all together in left hand.
    Grasp tag end of leader and wrap it over itself, tube, and end of fly line in six
    to eight turns. Move it from left to right toward the end of fly line and hold
    wraps in place with thumb and forefinger of the left hand.
3- Now pass the leader butt through the tube so the end passes back between
    the turns that already made. Then pull the ends of the mono-filament in
    opposite directions, then tightening them on top of the tube.
4- Then slip the tube out carefully while holding the wraps securely in place.
5- Pull tag end of leader butt until the turns begin to tighten. Then lubricate the
    knot and pull both ends of leader tight.
6- Finally cut the tag ends of fly line and mono-filament at the closest point to
    the knot.
here you can see how to tie Clinch Knot and also you can know about how to choose right fishing line