Euro Truck Simulator 2 mods:

When you downloaded your mod, firstly, you need to unpack it (if it is archived) and search the archive for file(s) with extension *.scs.

When you finally located the .scs file(s) just copy them to location: My Documents\Euro Truck Simulator 2\mod\.

That’s it, now you only need to launch the game and select files in game.