To deploy DMARC you must publish a DMARC record in DNS for your domain.
An example DNS TXT DMARC record should follow the format:
"v=DMARC1; p=quarantine; rua=mailto:dmarc-reports@yourdomain.com; ruf=mailto:dmarc-reports@yourdomain.com;"
Replace yourdomain.com with the client's domain name.