There is no additional link with a copy&paste howto - at least I did not know any.
You need to delete some indices to free up space: https://www.elastic.co/guide/en/elasticsearch/reference/6.7/indices-delete-index.html
After that configure your data retention and rotation in Graylog that it fits your available disk space or time you want to data to be persistent. Remember that for the second one you need the amount of disk space for that. https://docs.graylog.org/en/stable/pages/configuration/index_model.html#index-set-configuration
If that is done, make your indices read-write again:
curl -XPUT -H "Content-Type: application/json" https://localhost:9200/_all/_settings -d '{"index.blocks.read_only_allow_delete": null}'