Creating a Workshop Mod

Create your mod as normal as a subdirectory of the save directory \Mods folder
Open the Modding Utilities from the overlay main menu
Select the steam workshop uploader
Select your mod from the list
Click the "Create Workshop Id for Mod..." button. Your mod will now be associated with a workshop entry via a workshop.json file in the mod directory. You can now browse your item on steam workshop, though it will be empty. (6/25 there's a bug after create, click your mod again in the list and you should get the details after the create, will be fixed 7/1)
Optionally fill out the title, description and other fields. These can be edited in steam later, if you'd like.
Click the Upload Content... button. The contents of your mod folder will be uploaded.
Now users can Subscribe to your mod in Steam and restart Caves of Qud and get your mod content!