To contribute OSSC, fork, make your changes and send a pull request to the main branch.
Take a look at the issues for bugs that you might be able to help fix.
Once your pull request is merged it will be released in the next version.
To get things going in your local environment.
docker-compose up
Get your WordPress installation going. Remember your admin password. Once it's up, login to admin and activate the OSSC plugin.
Now you can make edits to the ossc.php file, and it will instantly take effect.