It's worth noting that if you have the non-definitive version installed, any mods you have should be automatically detected ...

This Shadows of Doubt guide will explain how to find and install mods and suggest a few interesting mods that can enhance the experience. Mods can add a variety new items, mechanics, and city ...

Pick the mod or map you want to have, and choose Subscribe to download it. Keep in mind that when you install certain weapon mods, they'll only be accessible by specific classes.

The mod's page here on Nexus Mods has instructions on how to install the mod, both locally and on a server, as well as how the controls work for the advanced building and editing modes.