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

What software/OS are you running on your NAS? If you’re running some goofy software on a private tracker your client might not be whitelisted.

Besides that - this NAS is attached to your home network I assume? Is it behind a router? Are the ports you’re using for torrenting port-forwarded?

What tracker are you testing this on? A bunch of trackers will have a “Connectivity check” that will tell you whether or not your client is connectable