Mono Loop Nail Knot

If you don't have a small tube handy, here is an alternate approach that allows you to substitute a short piece of monofilament or backing. Actually, backing can be easier to work with because it's amore supple.
1. Fold a short piece of mono and hold it parallel to the end of the fly line with the doubled end just beyond the fly line.

2. Wrap the butt section of leader 5 or 6 times toward the end of the fly line keeping the coils slightly loose (but holding them firmly in line with thumb and forefinger).

3. Insert the tag end of the leader through the doubled end of the mono and carefully pull the short piece of mono out through the coils you are holding in place. The mono will bring the tag end of the leader through the coils (away from the end of the fly line).
4. Discard the short piece of mono, and tighten the knot by pulling on the tag and standing ends of the leader (not the fly line) while making sure the coils stay in line.
5. Trim the tag ends.