You can use the UI or a simple REST API call to upload the tgz/tar.gz containing the Cookbook to a Chef repository.
Artifactory will automatically extract the relevant information from the metadata.json to later serve the index and respond properly to client calls. This metadata.json file is mandatory. If it does not exist in your cookbook, you can use a Knife command to generate it and then publish it to Artifactory. For example:
Publishing a new Cookbook
$ chef generate cookbook myapp $ knife artifactory share myapp tool