composer require --dev nextcloud/openapi-extractorTo avoid dependency and PHP version conflicts it is best to install the package to vendor-bin using https://github.com/bamarni/composer-bin-plugin instead.
The Workflow template repository has a template available: https://github.com/nextcloud/.github/blob/master/workflow-templates/openapi.yml
Afterward in your repository settings set the OpenAPI workflow to be required for merging pull requests.
Checkout the OpenAPI tutorial at https://docs.nextcloud.com/server/latest/developer_manual/client_apis/OCS/ocs-openapi.html to see how you can use openapi-extractor.
Make sure that you have xdebug turned off when generating OpenAPI specs, otherwise it can take multiple minutes instead of seconds.