Is there a good explanation of vpn options?

I am working my way through setiing up a VPN tunnel, and I am a bit stymied by the whole process. Is there a good explanation somewhere how this all works on my netgear router? Let's take for example L2TP. When I try to set that up, there are IKE policies, VPN policies, User setups, there is a VPN Wizard, etc. Nowhere can I find an explanation what I need or what is necessary or where the pitfalls are. For example, I have to select FQDN but then have to enter 0.0.0.0? That doesn't make sense. How does this all fit together? What are the dependencies?

For example, I have set up an L2TP user. So, if I connect with the right credentials, what is next? VPN pilicies? IKE policies? When I look at the VPN policies, I see that I can select an "Auto Policy", but then at the bottom I can (have to?) selct an IKE policy. What's more, I can select various parameters that seem to duplicate in the VPN policy and the IKE policy.

I admit that it is probably my lack of understanding that gets me in trouble, and that is why I am asking if there is a good explanation of all these different options and how they all intract to get to a stable and efficient VPN connection.

Appreciate any help.