Before you start:
https://support.google.com/a/answer/10032674?hl=en
Add the DMARC record
https://support.google.com/a/answer/2466563?hl=en
Setup relay for third party senders
https://support.google.com/a/answer/2956491
Defining the DMARC record
https://support.google.com/a/answer/10032169#zippy=%2Cdmarc-record-tag-definitions-and-values
Rollout for large orgs
https://support.google.com/a/answer/10032473
Finally, test the config: https://www.learndmarc.com/