From point 3 onwards, however, I can’t get any further because I can’t find the link to the instructions for setting up the VPN in the network manager. can someone tell me where to find these instructions?
First, in your VPN gateway VM, you would need to install a VPN plugin (such as NetworkManager-openvpn for OpenVPN connections): Projects/NetworkManager/VPN - GNOME Wiki!.
Then, you should see an option in the NetworkManager UI. See this for step-by-step instructions: Ubuntu/Mint OpenVPN on Network Manager - SlickVPN
(I don’t use or promote using SlickVPN. It was just the first search result.)