It's similar to the method of teaching children to tie a shoe. They need to practice it to get it right. Having someone physically there to show them is recommended.
There are very nice, useful videos on Youtube. You just need a tie, a mirror and a pair of these videos and in few minutes you will learn how to tie a tie.
The best way is to stand in front of the mirror with the person learning. You should both have a tie in hand. Have them copy what you do. There are also books that have good instructions if you do not know how to do it yourself.
First cross the parts of the tie so the shorter part is behind. Then bring the longer part around and behind the shorter part,bring it up and pull it up through the loop twice,bring it down through the knot in front.
