You need to run a server on the maschine, where you want to share the desktop. And you need to have client software on the N900 to actually connect to the PC.
Both shouldt share the same subnet to get the basic s working properly before you might want to extend this for worlwide usage. There might be no firewall tweaks necessary once the server runs on your PC, but you could for testing purposes stop the firewall.

1.To export the N900 to your other device, you need x11vnc application on the N900 installed. Start it from the menu, it will start and close to run in the background. Connect to IP_OF_N900:5800 from your vnc viewer on your PC. After you terminate the connection x11vnc will shut down without any notice and you have to start it again for the next connection.

2. To get the screen of your PC, a VNC server has to run on the PC. Some widely used vnc server applications are: TightVNC, RealVNC or UltraVNC. You need a vnc viewer like PresenceVNC or vncviewer to be installed on the N900. Connect to IP_OF_PC:5800 .