WAL Async Replication Question

List,

I've done   WAL async replication  between two  Postgre servers in Pirmary and Standby node as a test bed on two Virtual Machines on CentOS 8. Postgres12.

Now I want to do the same WAL replication on a  Production  Postgres  server to a new standby server.

Is there a way to do configurations for WAL async replication on  primary server without affecting the customers / applications that depend on this primary DB server?  

As it is a production server,  any changes we make to configuration files may affect the services right ?  or what  unseen problems can arise in this use  case? 

How to minimize the downtime / without service interruptions from the Primary server while doing WAL replication to a Standby server.

Any hints / directions most welcome .. How will geeks do this in this situation?

Yours,

Krishane