Just use your Dart knowledge and best practices, and communicate through Github Issues before making large changes.
Since there are two packages, we use melos to make things easier.
Install it:
dart pub global activate melos
Add overrides so that your creator package depends on your local creator_core package:
melos bootstrap
Make code changes, then test both packages (see melos.yaml for what it does):
melos test
Commit and create PR.
This is for the package owner.
Update version in pubspec.yaml in both packages. Update CHANGELOG.md in root folder.
melos clean to clear the local dependency overrides.
melos publish to dry run the publish.
melos publish --no-dry-run to publish the change.
Commit and push the change to github.