For adding subscriber you will need:
Network connectivity to Publisher
Cluster password
Add additional node license in Publisher Admin > Licensing > License File Upload
Add subscriber IP Address in Publisher Admin > System > Server
Start installing subscriber using install media
Do not choose First node during installation.
For more detailed steps follow the attached guide.
Post installation you will need to activate the services on the subscriber and then you may add it to Unified CM groups for cluster redundancy and also utilize its Voice media streaming resources.
If you add the subscriber to existing Unified CM group it will get added to the Device pools configured to use it.