How to decide and set dmarc to quarantine?

You may set it to quarantine first and see the emails which are getting quarantined in the quarantined mailboxes.

If the emails in the mailbox are looking genuine you can mark them safe and if all emails are not genuine and properly getting quarantined then you can set the action to reject.

DMARC reports tell you how others (f.e google) perceive your mail and how they handle your mail. (if reporting is enabled & setup).
How many they accept and how many they reject.

Based on that decision (if the mail you legitimately send out effectively is received and processed as valid e-mail) then it might be time to enable more severe restriction on the mail.  First to quarantine  and see reports on that, and if still satisfactory, enable dropping.

before changes let it run for quite some time where no changes were needed to keep the mail received. (btw. not all system do report on DMARC, some of the biggies do though).   If you send out a lot of mail you may want to handle a sample of the mail received on the other end.