That
howto is dated 2009, but I believe it must have been written years before, as it recommends using isakmpd.conf rather than the far more simple and easy to use
ipsec.conf(5), which became available in 2005.
To highlight the simplicity of the newer configuration tool, a popular
howto for ipsec.conf was titled
Zero to IPSec in 4 minutes. That proved so popular, that the OpenBSD Journal published a follow on article they titled
0 to IPSec in 4 minutes (To a Cisco!). I couldn't tell you whether the guidance in the latter
howto would be applicable to your environment. What it does highlight is that you will need a basic understanding of how IPSec works, how the two gateways nodes negotiate SAs and Flows, the capabilities of both gateways, and how to ensure that your traffic is properly encrypted between the two.
The
ipsec(4) man page is a good starting point on the road to understanding.