how to create a .torrent file

[user1]

If I have a file called yyyyyy.iso sitting in a folder on a webhotel, 
let's call the url address:

http://www.xxxxxxxx.org/tarballs/yyyyyy.iso

How do I create a .bittorrent file for yyyyyy.iso?

Please explain in details, as the documentation I have read so far has 
been a bit to complicated for me to understand (I am 70+ yrs old)

Do I need to give a port number somewhere, which I have read about.

Whatever I have tried has given errors and not worked.