Regarding not being able to find a tutorial for how to make a generic mod: the problem is that mods change the game in one way or another & every game is for the most part, unique. As a starting point, you'll need to make use of the ISteamUGC Interface, which contains the 'functions to create, consume, and interact with the Steam Workshop.' The specific details are too broad to cover here. The implementation information can be found on the previously mentioned page of the same name. This model typically requires an item server or use of the Steam Inventory Service for tracking ownership. The Curated Steam Workshop place content in a queue for customers to vote on and the developer must approve items for use.The Ready-To-Use Workshop allows anyone can upload content and players can download content without any curation.

Starting with the overview, you'll first need to decide on which workshop model you want to support: Searching for 'workshop' from the documentation homepage will bring up the references for the Steam Workshop Overview and the Steam Workshop Implementation Guide. The first thing you need to do is familiarize yourself with the Steamworks documentation.

Steam game wont download workshop mods