The knot is very simple to tie, but as the vid notes above you want the fly to be 'up side down' (point up) when you tie same. Gets really fiddly if you don't.
One other point. As to the knot (or any knot for that matter) being 100% that's a load of tosh. A few will get close, but for the most part the leader (just because of the knot) will loose upwards of 50% of its straight breaking strength.