FIRST things FIRST: download reshade... (reshade.me)Once reshade has downloaded and installed, RUN the file, and when prompted - direct it to your Witcher 3 file (DX12). 

SECOND install this mod (replace all shader FX files in the reshade-shaders file inside path:
C:\Program Files (x86)\Steam\steamapps\common\The Witcher 3\bin\x64_dx12

RUN your game, make sure it runs - once your into your game (moving around etc etc) - press 'HOME' key on your keyboard to open the reshade menu.

enjoy! :)

ps. sorry if there are any issues during installation - ive tried to explain as simply as i can!