I've had a young adult novel idea in my head for a long time, but I've had a pretty hard time finding out how to start it (i.e. I have the body and the end plotted out way more than the beginning). The story is about a girl whose best friend goes missing. The thing is, the story really begins three years after his disappearance, when she finds him in another world. I want their reunion to be meaningful to the reader-- but I can't do this if we don't have any background to their friendship or who her friend is. I was thinking of starting the story with a flashback to the day he disappeared, but I've been told that it's a bad idea to start immediately with a flashback. Any advice on how I could make it work? Or should I avoid the flashback entirely and try to come up with another way?