Modding a custom banner

[The Icelandic Viking]

I found a video with this, but it had no audio and it went back and forth so it was hard to follow.

Basically what I want to do is replicate the otherwise non-buildable NPC banners that the major factions have (Holy Nation, Shek and the UC) and make my own that I can place, either through the construction menu itself or shift+f12 (doesnt matter much, either way)

How would I go about doing so through the FCS, step by step? Thanks in advance, and sorry if it´s a stupid question, its been really difficult for a newbie like me to figure out.