Once I had in mind I wanted to make a Western trailer, I went to look for the music I was going to use. Given that it was heavily inspired by the recently released Red Dead Redemption 2 Trailer #3, I wanted something that would convey that sort of energy and tension.
I found a generic song that resembled the one on the trailer, so I used that one.
As for the video, I had already decided on a few scenes I absolutely wanted to add to the trailer, but I only finished picking the scenes once I had the song ready, as I "tailored" the scenes to fit the progression of the soundtrack I chose.
Once both of these parts were done, I only had to trim the videos to the required length, match key moments to the soundtrack, such as the axe hitting the puppet right when there's a "spike" in the soundtrack; as well as removing or leaving the video's audio in when it made sense to the trailer.