Well, a lot of it is just trying stuff out, but let's say you want to setup Navidrome because you read about it somewhere. My first step is always to go to https://search.nixos.org/options? and search for it, it'll show you the options available. If you want to know how it's implemented under the hood, press the "Declared in" link where it shows you the source code of the module, this can sometimes be helpful.

Other than that, read the wiki for examples, and remember that nix is a full language and not just a configuration, so you can keep it flexible.