Page 57 of the user manual describes the process...
Log into the router and take the "Forwarding" page. Specifically UPnP Forwarding. Assuming you keep your server on port 80, on the line for HTTP, select TCP, leave internal port at 80, make sure the IP address is the internal address of the server, and check Enable. That's it.
That was the easy part... Are you sure your provider will allow incoming connections on port 80? You may have to check with them and/or try different ports to get it to work. If you wind up using a different port, come back to the same router page and use one of the "generic" lines to forward the port.
BTW, you can use a regular web browser to test the connection externally. Point your browser to your public ip address and your chosen port and you should get a message that says something like "You are not using a standard Music01 protocol to access this server". That's a good sign. You got through.