DMARC is actually quite easy, because it’s just a TXT record.
There are a multitude of options, so best to choose some intelligent defaults to get you going.
Here is a good starting point:
_dmarc.example.com. IN TXT "v=DMARC1; p=quarantine; pct=100"
In essence, quarantine 100% of emails which do not pass SPF and DKIM.
If you’re using Virtualmin and you have set this as your default, you might also end up with something like this:
_dmarc.example.com. IN TXT "v=DMARC1; p=quarantine; pct=100; ruf=mailto:[email protected]; rua=mailto:[email protected]"
This can be slighly annoying, because your client might end up with emails from various systems that they do not understand. The point is DMARC systems co-ordinate their responses and when they feel something needs your attention they send you a summary. Furthermore special tools allows one to keep track of statistics.
Our advice: Start conservative, test, and take it from there.
https://mxtoolbox.com/emailhealth is a great testing tool.