Look at what is doing good, figure out what about it is doing good based on trial and error, then figure out why what I did worked or didn't so I can either repeat the process or replicate the process.

I transpose it into the way I would speak or write.

Like for example, if it says "A book about a mysterious dragon and a girl who is his rider."

I would say "This book is an (insert descriptor here) fantasy about a girl who finds out she is the bonded rider of a new mysterious dragon"

I would insert some details from that book to really flesh out the idea. Most likely this would either be on the second or third picture or the later half of a video.

For book content, I think of the book and either pull out a line that is scroll stopping, or I come up with something that relates to the book in someway.