Depends exactly what you're trying to create it for. I advocate for doing it during the build process rather than as a step after.

We open sourced a few tools that do it automatically for containers:

https://github.com/chainguard-dev/apko

https://github.com/chainguard-dev/melange

I've used the cyclonedx maven plugin, cyclonedx-bom on pypi, cdxgen for js, and cyclonedx-cli for various utilities (e.g. merging). All have worked great.