I started with qbittorrent, and the built-in search engine. I just activated all of them using the automatic settings and it was great. But the search engine named Jackett never worked, so I wound up going here:

https://github.com/qbittorrent/search-plugins/wiki/How-to-configure-Jackett-plugin

and after a bit I had it sorted out and working. Just make sure you follow the instructions step-by-step, and pay attention to the API key part. You don't have to go anywhere to get a key or pay anything, it's generated automatically.

Then I started looking into Sonarr. Since I had Jackett working, and Jackett will export Torznab links, transferring them over to Sonarr was pretty easy. Just copy the Torznab link and the API key into the Sonarr indexer settings, for each Jackett indexer you want to use in Sonarr. One thing I had an issue with was setting the content for each indexer, it defaults to some strange "5200", "5400" tags for some reason. If it can't detect the correct content, it won't pass the test, and you can't save the settings. I just opened up the content menu and clicked the box for everything, and then the test passed and the indexer was set up. It should be pretty straightforward after that. I had a few problems figuring out which settings and tags to use for each show (I like 4k UHD, but some shows are only 1080p, etc.) but I had things working nice after about a week.