I don’t know if there’s a “definitive guide” - it’s not that complicated to get a torrent client up and running. What kind of content are you looking for? Movies, Series, Music, Games, Books…?
Best is probably to try to get access to a decent private tracker, and an “easy” one - one with a bonus point system for seeding and uptime - that makes it much easier to keep a good ratio with a NAS, if you’re just permanently seeding everything you download, you’ll get points and “rise the ranks” of that tracker.
Once you’re a high enough rank on that tracker, you’ll get access to their “Invite Forums” where other private trackers advertise and give out invites to their trackers