There are two ways to install mods in the PC version of The Witcher 3 - manually, or through a mod manager.

Soon there will also be a DLL to add so that you would not have to modify the game executable itself. There are a ton of The Witcher 3 mods but you only see a handful if you are looking for a great experience. Exctract the mod archive to C:\Program Files (x86)\GoG Games\The Witcher 3 Wild Hunt\content\patch0\bundles then run the batch file for which ever variant you want. Download the mod, unzip, and put the modlimitpatcher.exe in the game's bin/圆4 directory.