DMARC
TLDR edit
You need to have SPF and DKIM properly established first.
(assuming your domain is 'example.com') Publish a DNS TXT record with this label:
_dmarc.example.com
and this value
v=DMARC1; p=none; rua=mailto:postmaster@example.com;
Background edit
IETF Specification edit
https://tools.ietf.org/html/rfc7489
Vendors edit
https://dmarcian.com/ is a place where you can get your reports nicely aggregated and reported.
See Also edit
Add a DMARC record in Google support