What kind of mod is it? This is what really determines how you'll need to go about it. If it's just assets, you drag and drop them wherever they would normally go on Windows. If it's in Steam, you can right-click the game, open Properties, and go to Installed Files. You'll then see an option to browse through its directory, and you just drag and drop as needed.
If it's something that goes in AppData, you'll follow the same steps, but go up two directories once your file manager is open. You should see a folder named "compatdata". Inside this folder is all of the support data for your games. You'll need to know the game's ID, but that's obtainable through Steam under Properties > Updates. Once you have that, open the game's directory, and navigate to pfx > drive_c > users > steamuser > AppData. Then, just drag and drop as needed.
If you're needing to use a custom DLL of some kind, you drop it into the game's directory like normal, and then you set a launch flag in Steam that tells the game to use it, instead of the one provided by the game's support data directory. Just replace "dllname" accordingly. It's not necessary to include the ".dll" extension, either.
Code:
WINEDLLOVERRIDES="dllname=n,b" %command%
If you want to make things a bit easier, you can also install Steam Tinker Launch.
https://github.com/sonic2kk/steamtinkerlaunch
This is a massive toolbox that makes it easier to use things like mod managers. trainers, ReShade, etc. Set up can be a little involved, though, so just be ready to read documentation.
