Mods must have two files to be uploaded to the workshop, modinfo.txt and logo.png.

Mod info is configured via the mod editor and saved to modinfo.txt, while you must create logo.png and place it alongside modinfo.txt within your mod.

In addition to those files, you'll want to start overriding base game files and folder. This is done by mirroring any folders and files you want to affect.

For example, to replace the railgun in the base game, add a file like so:
C:\Star Ruler 2\mods\<your mod>\data\subsystems\supports\weapons\Railgun.txt
Your mods copy of Railgun.txt will be loaded in place of the base game's copy.

You may also add new support weapons by placing a new file into that same folder. The same approach works for nearly every other folder and resource.

Keep in mind your mod may be played by Linux users, so your folder and file names should be treated as case sensitive.