When you download a mod file it is usually EXE or ZIP/RAR filetype.
EXE
When the download is finished, find it. Just double click on it and when it runs it will install itself.
ZIP/RAR
Download the mod file and open it with WinZip/WinRAR/7z. When you see different type of files close the compressed file and copy it to your game mods folder. When you see more compressed files, then extract them, usually is written unzip or open into the compressed file title. You can find your game mods folder at:
Documents/My Games/FarmingSimulator2015/mods
Destination of the folder may be different in different Windows and the German version folder names are slightly different. If you have any questions you can contact me.(contact info at the contact page). Have fun using mods.
