There’s no quick answer here
It’s hard to answer without knowing how familiar you are already with Juju and charming.
There’s some existing documentation / information on writing k8s charms, eg
plus a number of other posts in the Kubernetes category on advanced configuration, storage etc
Here’s there’s a generic charm writing guide:
Note though that there’s also a charm operator framework which is being used to write charms moving forward:
Hopefully some of the above links are useful. Of course, you will also need to know about fluentd and how that all needs to be set up and configured.