Running as a server is very simple, just choose a port to listen on (or leave it as the default value) and click to start. The server will remain listening until the client connects (or you cancel the process).
Once the client connects the server will start the repair process. The server stops listening for new connections after the first client is connected. You will be kept informed during the operation by both a progress bar and status text.
There is a special case when passive mode is enabled. Here, after the client has connected, the server switches roles to repair it's own copy of the file based on the client's version. This allows you to get files repaired even when the other user is unable to receive connections (e.g. behind a firewall or on a NATed connection).