In short, you download a microscopic file called a torrent, and then feed it into Torrent program like BitTorrent or Azereus (sp.?), which then starts downloading the actual file for you, in bits and pieces from fixed sites and others who have downloaded it. Once you've downloaded the mod file, you help others download it by hosting it. All you have to do is leave it in the directory you downloaded it to.
As for download means, have LucasFiles host both a torrent and a zip. Or include the torrent in the zip. That'll work too
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"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.