Open putty and go to connection > Data

Provide username as opc

Go To Auth and upload the private key pair which was generated as mentioned in

https://dbatracker.com/2020/05/26/connect-to-compute-instance-using-putty/

Provide the public ip of the VM instance in Tunnels and port 5901 as mentioned below and click add

After adding it should look like this

Now save the session with public ip and name

If you try to connect you may receive this error. It says private key in old PEM format is not recognised

To change the format of the private key to ppk open putty key Generator and click load.

Provide the location of the private key and click open

Your private key is now imported. Save it in ppk format

Save private key and provide new name. I am not providing pass phrase but you can provide if you want extra security

Give the name

Now your private key is converted in ppk format

Lets use this one in putty now. Remaining settings will be same.