Not many people wear bow ties today and even less know how to tie one – I have liberated this excellent article on how to do this from another website…

bow01.gif 1) Place the bow tie around your neck, situating it so that end “A” is about two inches longer than end “B”. bow02.gif 2) Cross end “A” over end “B”.
bow03.gif 3) Bring end “A” up and under the loop. bow04.gif 4) Now double end “B” over itself to form the front base loop of the bow tie.
bow05.gif 5) Loop end “A” over the center of the loop you just formed. bow06.gif 6) Holding everything in place, double end “A” back on itself and poke it through the loop behind the bow tie.
bow07.gif 7) Adjust the bow tie by tugging at the ends of it and straightening the center knot.    

Source: http://www.tie-a-tie.net/bowtie.html