If the client is properly registered with the server, all you need to do is to set an appropriate threshold for remote logging in the client's configuration file, and give the IP address of the server (if not already compiled in). Of course, the client must be compiled with the --enable-network=client switch.
Example for client configuration:
[Log] # # Threshold for forwarding to the log server # ExportSeverity=crit [Misc] SetLogServer=IP address
Example for server configuration:
[Clients] # # Register a client to allow it to connect # Client=client.mydomain.com@salt@verifier