Publishing
Mods can be installed locally should the mod not be available on Steam, you want a specific version, Steam is refusing to serve the correct version, or really any other reason. Simply copy the entire folder containing the .dll (not as a zip or other archive) into the mods\Local folder within the game's files, creating it if it does not exist. The mod's name in-game will be the name of the folder, verbatim.
Other than mentioned above, the mod will function identally to one installed via Steam. If you want your mod to behave different upon a crash, see the development section for more info on the mods\Dev folder.
Nothing says you have to publish to Steam (and there are valid reasons not to), so you can simply place the folder containing your mod files into an archive (.zip or similar) and send it to whoever you which via whatever platform you desire. The person on the other end needs to simply unpack the archive and the game will load the mod.
You will find the uploader in your Steam library under the name of Oxygen Not Included Uploader. Make sure to enable showing tools in the library first:
The uploader tool lets you add and modify your existing mods. You need to prepare:
- Mod files (follow instructions from the chapter above: the folder containing the
.dll, not zipped) - Official mod name (how you name the mod on Steam is how the players will see it in-game)
- Mod description (using Steam formatting)
- A thumbnail image
After filling all the fields, choose applicable tags and hit Publish!. Keep in mind, the tool is not perfect and sometimes throws a moody fit - keep a backup of your lenghty description in notepad to avoid excessive frustration.
If it is your first contribution to the Steam Workshop, your submission will be set to private and Steam will prompt you to accept terms of conditions. Once you do that, go to your mod page and then set the visibility to public.
That's it! When you need to edit the mod, you can tell the uploader to update only specific fields using the checkboxes next to them.
Steam tends to have major issues with mod updates -- sometimes it will continue serving an outdated version for some people, and while there are some tricks to go around that (resubscribing, deleting the mod folder, wiping cache), there is no 100% viable way. Therefore, it's a good idea to not update too frequently and wait for more significant releases; or to also provide a link to downloadable mod files that can be installed outside of Steam.