The links masterneme posted are very helpful. However, to get started with Steamworks and getting the hang of Steam Callbacks, I also recommend the tutorial I made. As a lot of the stuff you learn there are similar to how the workshop does it.
https://wiki.unrealengine.com/Pause_Game_On_Steam_Overlay_Active
Looks like it might help. I didn’t use it for my game, because I got lucky that all the files I need to mod my game are in a very basic format.