1. Get a trailer, preferably secondhand.
2. Fill it with books.
Okay, just kidding. It's actually even easier than that (and free!) to make your own book trailer. Here's how.
1. Find some images (including your book's cover image) that you like, and pick a bit of text to go with each image. The text could be exerpts from the book, praise, brief author bio, or general "tease"s or hooks to intrigue potential readers/buyers.
2. If you use Windows, the former Windows Movie Maker has been replaced by Photos, which is also free. If you use Apple, iMovie will do.
3. Create a new movie project in either platform. In Microsoft, add images to your project library and drag and drop them to your storyboard, where you can set duration (e.g. 5 seconds) and add text. In Apple, import images/videos, drag and drop into the film reel, and add captions.
4. In my case, I liked the captions in Microsoft Photos but hated the special effect, so I had to go to someone's house that had a Mac and use their iMovie to trim off the special effects (the drag-and-trim function is amazing, like virtual scissors on film reel). You can also add audio here, but I skipped that because I was planning on using Youtube's license-free music.
5. In Photos, export/share to create a video file to upload to Youtube. IMovie shares directly to Youtube. If you want, you can add music to your video using video editor in Youtube, and then download your video again for future use (complete with license-free music) using y2mate (https://y2mate.com/youtube/). (No funny apps or programs to install--enter your video address and it lets you download the mp4 file.)
6. My results are below. The whole thing took one manic evening, though the watercolors were done years ago, and I'm really glad they've finally been put to good use!