First, connect the existing shared storage to the node that you will add to the cluster. Then, configure the appropriate network settings - DNS, gateway, AD, etc. Run the Failover Cluster Validation Wizard to check if the server that you will add is fine
and will not cause issues. If possible, take the SQL Server resource offline and run all of the tests. If not possible, select only the tests you want to run and exclude the storage tests. You can then add this server as a node on the Windows Server Failover
Cluster. Once the server has been added, run the Add Node option to install the SQL Server 2012 binaries on this server. This will join this server as a node in your SQL Server failover clustered instance.
Keep in mind that if you intend this server to act as a standby node on all of the SQL Server instances running on your Windows Server Failover Cluster, you will have to run the Add Node option as many times as the number of SQL Server fasilover clustered