It’s actually not that hard. The hardest thing is to think about your scenario and to pen your storyboard. Once you know what exactly you want your scenes to look like, you just go to WoW Unit Viewer and get the unit models you want in the poses you need to have.
Then, you put them on a video editing software and you literally build your video.
Nixxiom, whom I think is one of the best at it, even made a video explaining how he works : https://youtu.be/N-hBogx-bQ4