I DONT MAKE TRAILERS SORRY NOT SORRY
First Step: Think about it!
If you want to make a really good trailer, first think about the main story of the book. For example:
The story is about a girl that just found her dad dead. But she wants to know WHY. While searching for answers, she meets a guy all of a sudden and they fall in love. They have adventures together but then they brake up (because of reasons, you pick why, I don't know lol) and she moves far away. She meets another guy. But you know what is the worst: She still hides what happened to her dad and continues to search for answers.
What will happen? Will she learn why her dad died? What about the guys? What if they are linked to her dad and not saying it?
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See? You need the MAIN IDEA or the MAIN STORY for the trailer. Think about it!