Tunnels are configured for PuTTY sessions associated with a particular host. You will find it convenient to save a session for the tunnel.
To get to the Tunnel section of the PuTTY configuration, you need to click the
+ next to SSH to open its submenu. Then click on Tunnels, which should result in window like this one.
Enter the VNC port in the Source port box. In the Destination box enter the compute hostname followed by a colon and the port number. That should result in a window like this one.
Finally, click the Add button, which should result in a window like this one.
Now that you have a port forwarded, you are ready to get on to step 4, connecting your VNC client to your running VNC server.