How to view user session using vnc

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http://askubuntu.com – I want to view (realtime) the current session on a remote Ubuntu system. I need this for instruction and troubleshooting purposes. I have ssh access to the server with root access. So I've tried installing vnc4server on the server (apt-get install vnc4server), then login as user (su - username) and starting a vnc server (vncserver :1). But I am kind of stuck with how to use vncviewer. Can anyone give me a step by step instruction on how to set this up with me only having ssh access and root access to the server? I've heard of X11 forwarding being a feature of X window system but I don't get h (HowTos)