The easiest way is to install the Nexus Mod Manager and download and install the mod using that.

I prefer installing mods manually which is pretty easy and straightforward in W3.

1) Download the mod and extract it (usually the mod is compressed)
2) Create a "Mod" folder in your Witcher 3 folder (e.g.: "C:\Games\The Witcher 3 Wild Hunt\")
3) Copy the folder you extracted in 1) to "Mods"

Thats all you need to to, if you want to install more mods it is recommended to create a "mods.settings" file.

Why are you telling me this?

I saw it as a sort of addendum to your previous post, but it was more addressed to reneck1st than you to be honest.

I knew that, but felt the need to bring to your attention that when you hit 'Reply', you are by definition addressing the person you reply to; like looking at the person you are speaking to -- you don't talk to one person while looking at another. Same goes for forum replies.