I’ve just finished setting up Virtualmin and have started creating virtual servers for domains and mail accounts. I’m now looking into backups, and would like to know what is a good backup process to follow, such as:
What you should backup all depends on what data is important to you 
I’d suggest starting by going into “Backup and Restore” in Virtualmin, and having it backup all your Virtual Servers and configuration at least once a day. Using the Backup and Restore menu, you can schedule backups to run at certain intervals.
A common thing to do is a full backup run once a week, and incremental backups running once a day.
-Eric