https://github.com/rusefi/fw-custom-example is an example of a custom rusEFI firmware repository. Just fork it, edit ``meta-info.env`` with name of your board and let GitHub Actions do the rest!
https://github.com/rusefi/fw-custom-paralela and https://github.com/rusefi/fw-custom-core8/ are the best example as of right now. https://github.com/rusefi/rusefi/tree/master/firmware/config/boards should be another source of inspiration.
Alternatively open "Actions" in your repo similar to https://github.com/rusefi/fw-custom-example/actions/workflows/update-rusefi-reference.yaml click "Run Workflow"->"Run Workflow"