OK, I think I know where you're confused. Lets assume that vncserver on the remote box is on screen 1, so we're using port 5901. In Putty, you want the local port to be 5901 and the remote port to be 188.8.131.52:5901 only replace 184.108.40.206 with the LAN IP address of the remote machine.
So on my network, I have an IP address from my ISP (the WAN IP) and my Linux box has an LAN IP address (192.168.1.50) that is non-routable. So to make the SSH connection, I connect to the WAN IP address (which is forwarded to my Linux box) but to make the tunnel I use the LAN IP address. So on my Putty, the Destination port is 220.127.116.11:5901. If the ssh connection is OK, I then fire up a VNC client (on the local machine) and point it at localhost:1 and the connection is made.