IPSEC is an aging protocol. In many ways, it is inferior to many of the more current VPN protocols, such as OpenVPN and Wireguard. For these reasons, there is no official wiki page explaining how to configure it.

However, these unofficial discussion threads and guides should help you get started with configuring IPSEC.

Tomato Forum Thread: Tomato and IPSEC. Why Not?
https://www.linksysinfo.org/index.php?threads/tomato-and-ipsec-why-not.38344

David Ferguson: howto-tomato-l2tp-ipsec-server.markdown
https://gist.github.com/dferg/50c378701a192bc8ce6a

(Polish) Tutorial by Shibby (Tomato fork Developer): IPsec on Tomato - Site-to-Site IKE PSK with Draytek

https://openlinksys.info/forum/viewthread.php?thread_id=20790

(In Polish. Google translator should help).

Note that some steps may differ from what is currently required, and some modules may already be included or differ in FreshTomato.

This is almost unavoidable since these are old tutorials/HOWTOs.