At the moment for the game Euro Truck Simulator came a lot of different modifications that in some way change the game, making it better, more interesting and varied. These mods can change absolutely different aspects of the game: graphics, gameplay, balance, models, landscape, tasks, interface, and so on. Of course, each modification requires proper installation. For most mods you only need to copy the file to the correct folder and confirm the activation of this mod in the game.
First you need to download the mod itself. Clicking on the mod's page, you save it to your computer. Mods are downloaded in the archives, so after downloading, mod need to open an unarchiver like WinRAR or 7-zip. The archive will have exact mod files and readme files, which, in fact, is the installation manual of the mod. In most cases, the files should be unzipped into the folder:
My Documents/Euro Truck Simulator 2/mod
If the file with such name already exists, you will first need to make copies of them (in case of conflict), then replace the existing files with new ones.
Once this is done, you need to run the game. In the main menu on the right side of the screen there is a button "Modifications", you should press it. You will see the modules modifications (files that are dumped in the mod folder). Double-clicking on the module activates it.
That's all, the mod is installed and activated, it is now possible to load (if the mod doesn't require starting a new game) and continue to play in the new ETS 2.