How to Scenarios?

As the title says, I've been looking around the forums, but to little avail on how to create my own scenario.

More specifically, how to make the AI race each other in the context of a scenario. Any answers regarding how to create any sort of scenario are welcome too though. I recall seeing a HighDef video on how to create a race with the AI, but it was with the old world editor, not the new one.

The information I've seen on the new world editor and how to create scenarios from other users seems kinda scarce and difficult to understand how it works. So if anyone could provide a comprehensive guide on how to properly create something from it, that will greatly be appreciated!