I'll try to hit the highlights.
Basically you signup for a Usenet service. After many days reading I landed on using
https://usenetserver.com/en/index.php?utm_expid=655196-5 this is a great site that explains newsgroups and the nzb file - an xml file that acts much like a torrent file -
http://www.newsgroupservers.net/. If you look on that last link you should fine a link to subscribe to the usenetserver for $10/month.
If you are not familiar with newsgroups at all a little reading will go a long way. The biggest advantage I've found is that they are safe, secure and you get great quality.
Once you've subscribed a few additional things are needed as the usenetserver only provides the service much like an Internet Service Provider only provides the Internet services you pay for. To get access (upload/download/search, etc.) we need two items.
1) A site that indexes public newsgroups (e.g. NZBMatrix, Newzbin2.es, etc. - these sites are a one-time lifetime subscription for VIP services, which is around $10 - free sites exist too, but don't have some features that could make the experience better)
2) An application that can import .nzb files, download, extract, etc. the file you are requesting (for MAC Unison - Windows has NewsLeecher - many more apps for both MAC and Windows but these seem to be the most popular - around $20 - $30 depending on OS one-time charge). However, SABnzbd is free and is a newsreader as well. The difference with SABnzbd is it is a bit more involved to setup and utilizes Sick Beard and CouchPotato for a total solution/experience.
I recommend going here -
http://www.totalhtpc.com/the-comple...abnzbd-sick-beard-couchpotato-and-headphones/ and clicking on the ultimate Usenet guide link, which is a .PDF providing instructions on setup (I haven't tried headphones).
Installing SABnzbd and CouchPotato is straight forward .dmg installers. With SickBeard a little more is involved (python, cheetah and git). It still only takes five minutes or so to install - so not difficult at all. So just a little overview of all three.
SickBeard is in my terms a PVR. With SickBeard (once configured with the Usenet credentials and indexing sites) you can search for any TV show. For example I have Person Of Interest as I show I want it to find among many others. It will find the shows I want and all subsequent episodes - much like a DVR as it downloads the shows automatically once available, which is minutes after the show has aired. CouchPotato does the same but for movies vs. TV shows. They both search the Usenet servers and indexing sites for nzb files. They send those .nzb files to SABnzbd to download. SABnzbd then downloads, extracts, and confirms the health of the download and will update XBMC or PLEX. When it's setup it takes all the pain out of searching index sites and within newsreader clients for files. SABnzbd will extract .rar and create the .mkv file for you. For me it truly makes the HTPC experience real - one that you could cut the cable or satellite completely out (so you don't need to get Unison for the functionality we're speaking about here).
Today I use my Macbook pro for my HTPC, which isn't practical. I have plex media server running on my iMac - so I want to get a dedicated machine now with the amount of use XBMC and PLEX are getting.
Hope that helps! And thank you for the update!