We do not yet have an up-to-date modding guide on this new wiki. For now, you can refer to the following Old Wiki guides, bearing in mind they are for Classic Cosmoteer and not the modern Steam release.
Accessing the old wiki will likely produce an error talking about an SSL warning. This is because the signing certificate has expired for the old wiki due to its age. The warning can be ignored, and the website remains safe to visit.
Creating your own mod
A great place to start is the built in example mod.
You can find it in the game under mods or under directly in the file system under SteamLibrary\steamapps\common\Cosmoteer\Standard Mods\example_mod
In this folder is a mod.rules file. This acts as the entry point for the mod and is highly commented.
Once you are ready to create your own mod and want to play around a bit, open the game>Mods Manager>burgermenu>Show Folder>User Mods.
There create a new folder (e.g. your modname) and create your own mod.rules there.
When you are finished coding start Cosmoteer in develop mode and then you can find your mod next to the standard and downloaded ones.