click on the GENERATE NEW RECORD button to generate a new DKIM record;
publish the DKIM record in the DNS; DNS propagation might take up to 1 hour before the record becomes accessible; Host/Name/Alias: @ Time to Live (TTL): 3600 or default Content/Value/Answer/Destination: value that google generated
once the record is accessible, click on the START AUTHENTICATION button;
click SAVE to complete the authentication process.