Get the needed CNAME or TXT DNS record from Google. Then, enter the entries to your DNS provider and wait for propagation.
Google guide for verification: https://cloud.google.com/identity/docs/verify-domain-txt
How to add DNS records in Z.com: https://my.web.z.com/index.php?rp=/knowledgebase/47/Where-do-I-edit-my-domain-DNS.html