S3LightFixes is a descendant of Waza-lightfixes, which itself is a descendant of Lightfixes.pl by vtastek. All three applications are designed to make ESP files which adjust the lighting values from all mods listed in one's openmw.cfg.
In other words, make light gud. What sets this version apart is that it's a standalone binary application, instead of piggybacking off tes3cmd. The changes it makes are the same as the previous two, but with additional conveniences like automatic installation and support for portable installs of OpenMW, and itself having greater portability.
Starting from version 0.1.9 and onward, S3LightFixes is now cryptographically signed using SigStore on all platforms. Release packages include the necessary .bundle files to ensure the cryptographic signatures for yourself.
Verifying the binary on a linux-based system:
cosign verify-blob ./s3lightfixes --certificate-identity-regexp="https://github.com/magicaldave/S3LightFixes/.github/workflows/" --certificate-oidc-issuer="https://token.actions.githubusercontent.com" --bundle ./S3LF-ubuntu-20.04.bundle
Replace ubuntu with macos or windows as needed for your target platform.
Just download the executable for your OS and run it however's most convenient. Double-click it or run it through the terminal or whatever.
That's it. Just go check for the S3LightFixes.omwaddon in the launcher. If it's there, everything worked. If it's not, a giant error message should have appeared in your face.
If you have to do anything else, if you think you have to do anything else, if it's not Just Working with almost no input from you at all - ask for help.
That's it.
Go away.