Installing mods for farming simulator is really simple and it is same for all version. First of all you need to download mods for correct version of farming simulator.
Be careful with this. If you have farming simulator 2015 you obviously need mods for farming simulator 2015. Mods for older versions doesn’t work with new one.
Mods can be found like .exe (auto install) or .zip files (copy+paste).
If your mod you just downloaded is .zip file (you don’t need to unzip), find this path:
users/yourname/documents/my games/farming simulator 2015/mods/
Just copy mod .zip file to your farming simulator 2015 mods folder. That’s it! You’ve just installed your first mod. Go in game and purchase it from in-game store. Have a nice play!