Use nag-toolkit library, call declare_lints!() macro and export resulting ::register() function: see example-nag-linter/src/lints.rs as an example.

You can provide a crate with lints for the users of your library or just write them in the same crate as your linter.

nightly-rustc documentation, especially part about LateLintPass is extremely useful when writing lints.

Use nag-driver library, call nag_driver::run() or nag_driver::Driver::with_callback() and then .run(): see example-nag-linter/src/main.rs as an example.

Install cargo-nag binary. Set CARGO_NAG_LINTER_DIR environment variable to the absolute path to your linter source code, then run cargo nag. It will compile and run a linter for you.