You probably didn’t find tutorials because it’s not generally something you can do by yourself. You are not authoritative for your network reverse DNS, your network provider is (hosting provider, colo, cloud service, whatever).
Some providers will delegate it to you, and then you just create reverse records as usual. In Webmin you just click the Create Master Zone and then switch to Reverse (Addresses to Names) and fill in the appropriate values.
Some will just allow you to set the reverse record for your IP(s) (which is probably easier and should be fine).
It doesn’t matter what your PTR record is, as long as you have one and it resolves both directions (i.e. you lookup the IP, it resolves to a name, and when you lookup that name it resolves back to the same IP you started with…nothing else matters…it doesn’t have to be “your” domain name or in a zone you manage).
There are various docs we’ve written over the years. This is the closest I can find to the delegation scenario, but is maybe deeper than you need to go: BIND DNS Server - Webmin Documentation
But, any BIND docs should work. We aren’t changing the way DNS works, we just configure BIND. There is nothing Virtualmin-specific about PTR records.