I want to write a warfare guide for the game. However while I have looked around I have not found any sort of useful template or structure such guide, do anyone know where I can find a good structure or similar.

Clausewitz and Sun Tzu would probably make good models for that. Aside from that, it depends what you intend to write, I guess. Should it become a comprehensive guide? Then it should cover general combat rules, general tips (including terrain etc.), trait and unit overview, army compositions, how to prepare for war, specific strategies, tech tree beelining, how to maximize your war score, and maybe a few specific examples. How to structure this in the best way for Humankind isn‘t an easy task, as there are so many interrelations between the sections/topics. And that eras change so much how warfare works doesn‘t help either… I would recommend to start with more general parts, as we‘ll probably see some balance changes when it comes to units over the next few months. Good luck!

I’m not sure if this would be helpful, but I personally breakdown warfare in strategy games into three categories:

1:  The grand strategy.  This part covers:  Who am I going to war with?  Why am I going to war? Do I need land, are there resources I need, do I need to slowdown a top opponent, etc.?

2:  How am I going to conduct this war?  Where do I want the major battles to be fought?  Is there a nice hill overlooking a river?  What kind of an army should I bring?

3:  Tactics, what do I actually do inside battles?

I hope this helps, at least a little.  It’s great to see people wanting to make guides for this game!

Here is a start for a more specific subject: https://www.games2gether.com/amplitude-studios/humankind/forums/173-strategy-guides/threads/45319-how-anti-aircraft-seems-to-work-based-on-testing?page=1#post-338026