Well unlike in the past. The new and upcoming SweetFX version will probably work if you don't change anything in the game. That does not mean you can just leave all the games setting (post-processing jod's) running. So switch off those setting in the game that are going to be handled trough SweetFX. If you're new to shaders maybe it's handy to know this is a general rule of thumb! For the witcher3 you should probably turn off the following techniques:
leaving them on will mean you're torturing you're PC, thus resulting in bad performance since you're machine is calculating the same thing twice. It will probably also make you're game look more ugly instead of nice and sparkling. So if you download a setting and see that blooming is used, you now know you need to turn the blooming off somewhere in the video options of you're game.
If you copy and past some setting file to you're text editor of choice but notice everything looks messed up. That's because you're editor is using a different codepage, also referred to as file encoding. Check if you're editor can change it to ANSI. Basic notepad can be used but I do not recommend using it. My editor of choice is Notepad++ due to the many advanced programming features.