How to install windows through USB?

Alright so I just got a new laptop that uses windows 10 and I absolutely hate it. I want to install windows 8.1, which was what I was using on my previous laptop without issue. I heard you can do this for free by downloading the files from Microsoft onto a USB and then installing it that way, but how? I've seen a few videos of this but I'm still lost. Any help would be appreciated. I'm not versed in tech-lingo so please try to explain everything well.